PSAT Math : Operations with Fractions

Study concepts, example questions & explanations for PSAT Math

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Example Questions

Example Question #102 : Arithmetic

Define an operation \(\displaystyle \bigstar\) as follows:

For all real \(\displaystyle a, b\),

\(\displaystyle a \bigstar b = 2a - 3b\).

Evaluate \(\displaystyle \frac{4}{5} \bigstar \frac{3}{10}\).

Possible Answers:

\(\displaystyle -1\frac{4}{5}\)

\(\displaystyle -\frac{21}{50}\)

\(\displaystyle 1\frac{11}{25}\)

\(\displaystyle \frac{7}{10}\)

\(\displaystyle -\frac{1}{10}\)

Correct answer:

\(\displaystyle \frac{7}{10}\)

Explanation:

\(\displaystyle a \bigstar b = 2a - 3b\)

\(\displaystyle \frac{4}{5} \bigstar \frac{3}{10} = 2 \cdot \frac{4}{5} - 3 \cdot \frac{3}{10}\)

\(\displaystyle = \frac{2}{1} \cdot \frac{4}{5} - \frac{3}{1} \cdot \frac{3}{10}\)

\(\displaystyle = \frac{8}{5} - \frac{9}{10}\)

\(\displaystyle = \frac{16}{10} - \frac{9}{10}\)

\(\displaystyle = \frac{7}{10}\)

Example Question #102 : Fractions

Define an operation \(\displaystyle \triangleright\) as follows:

For all real \(\displaystyle a, b\),

\(\displaystyle a \triangleright b = 7 - ab\).

Evaluate \(\displaystyle 5 \triangleright \left ( - \frac{5}{3}\right )\).

Possible Answers:

\(\displaystyle 11\)

\(\displaystyle 15 \frac{1}{3}\)

\(\displaystyle -1 \frac{1}{3}\)

\(\displaystyle 10\frac{2}{3}\)

\(\displaystyle 4\)

Correct answer:

\(\displaystyle 15 \frac{1}{3}\)

Explanation:

\(\displaystyle a \triangleright b = 7 - ab\)

\(\displaystyle 5 \triangleright \left ( - \frac{5}{3}\right ) = 7 - 5 \left ( - \frac{5}{3}\right )\)

\(\displaystyle = 7 - \frac{5}{1} \left ( - \frac{5}{3}\right )\)

\(\displaystyle = 7 - \left ( - \frac{25}{3}\right )\)

\(\displaystyle = \frac{21}{3} + \frac{25}{3}\)

\(\displaystyle = \frac{46}{3}\)

\(\displaystyle =15 \frac{1}{3}\)

Example Question #2 : How To Subtract Fractions

Define an operation \(\displaystyle \Cup\) as follows:

For all real \(\displaystyle a, b\),

\(\displaystyle a \Cup b = a - b^{2}\)

Evaluate \(\displaystyle 5\frac{1}{2} \Cup 1 \frac{1}{3}\).

Possible Answers:

\(\displaystyle \frac{1}{6}\)

\(\displaystyle 3 \frac{13}{18}\)

\(\displaystyle 17\frac{13}{36}\)

\(\displaystyle \frac{5}{7}\)

\(\displaystyle 4\frac{5}{18}\)

Correct answer:

\(\displaystyle 3 \frac{13}{18}\)

Explanation:

\(\displaystyle a \Cup b = a - b^{2}\)

\(\displaystyle 5\frac{1}{2} \Cup 1 \frac{1}{3} = 5\frac{1}{2} -\left ( 1 \frac{1}{3} \right )^{2}\)

\(\displaystyle = \frac{11}{2} -\left ( \frac{4}{3} \right )^{2}\)

\(\displaystyle = \frac{11}{2} -\left ( \frac{4^{2}}{3^{2}} \right )\)

\(\displaystyle = \frac{11}{2} - \frac{16}{9}\)

\(\displaystyle = \frac{99}{18} - \frac{32}{18}\)

\(\displaystyle = \frac{67}{18}\)

\(\displaystyle =3 \frac{13}{18}\)

Example Question #2 : How To Subtract Fractions

Define an operation \(\displaystyle \bigstar\) as follows:

For all real \(\displaystyle a, b\),

\(\displaystyle a \bigstar b = \frac{1}{2} a - \frac{1}{3} b\).

Evaluate \(\displaystyle 15 \bigstar 10\).

Possible Answers:

\(\displaystyle 46\frac{2}{3}\)

\(\displaystyle 71\frac{2}{3}\)

\(\displaystyle 10\)

\(\displaystyle 0\)

\(\displaystyle 4 \frac{1}{6}\)

Correct answer:

\(\displaystyle 4 \frac{1}{6}\)

Explanation:

\(\displaystyle a \bigstar b = \frac{1}{2} a - \frac{1}{3} b\)

\(\displaystyle 15 \bigstar 10 = \frac{1}{2} \cdot 15 - \frac{1}{3} \cdot 10\)

\(\displaystyle = \frac{1}{2} \cdot \frac{15 }{1}- \frac{1}{3} \cdot \frac{10}{1}\)

\(\displaystyle = \frac{15}{2} - \frac{10}{3}\)

\(\displaystyle = \frac{45}{6} - \frac{20}{6}\)

\(\displaystyle = \frac{25}{6}\)

\(\displaystyle = 4 \frac{1}{6}\)

 

Example Question #2 : Operations With Fractions

Define an operation \(\displaystyle \Cup\) as follows:

For all real \(\displaystyle a, b\),

\(\displaystyle a \Cup b = \frac{3}{5} a -b\)

Evaluate \(\displaystyle \frac{1}{3 }\Cup \frac{1}{10}\)

Possible Answers:

\(\displaystyle \frac{1}{5}\)

\(\displaystyle \frac{1}{10}\)

\(\displaystyle -\frac{41}{150}\)

\(\displaystyle \frac{7}{50}\)

\(\displaystyle -\frac{1}{50}\)

Correct answer:

\(\displaystyle \frac{1}{10}\)

Explanation:

\(\displaystyle a \Cup b = \frac{3}{5} a -b\)

\(\displaystyle \frac{1}{3 }\Cup \frac{1}{10} = \frac{3}{5}\left (\frac{1}{3} \right ) - \frac{1}{10}\)

\(\displaystyle = \frac{1}{5}\left (\frac{1}{1} \right ) - \frac{1}{10}\)

\(\displaystyle = \frac{1}{5} - \frac{1}{10}\)

\(\displaystyle = \frac{2}{10} - \frac{1}{10}\)

\(\displaystyle = \frac{1}{10}\)

Example Question #1 : Operations With Fractions

Define an operation \(\displaystyle \triangleright\) as follows:

For all real \(\displaystyle a, b\),

\(\displaystyle a \triangleright b = a^{2}- b^{3}\).

Evaluate \(\displaystyle \frac{4}{5} \triangleright \frac{4}{5}\).

Possible Answers:

The correct answer is not among the other responses.

\(\displaystyle -\frac{4}{5}\)

\(\displaystyle \frac{4}{5}\)

\(\displaystyle -9\frac{3}{5}\)

\(\displaystyle 9\frac{3}{5}\)

Correct answer:

The correct answer is not among the other responses.

Explanation:

\(\displaystyle a \triangleright b = a^{2}- b^{3}\)

\(\displaystyle \frac{4}{5} \triangleright \frac{4}{5} =\left ( \frac{4}{5} \right )^{2} - \left ( \frac{4}{5} \right )^{3}\)

\(\displaystyle =\left ( \frac{4^{2}}{5^{2}} \right ) - \left ( \frac{4^{3}}{5^{3}} \right )\)

\(\displaystyle = \frac{16}{25} - \frac{64}{125}\)

\(\displaystyle = \frac{80}{125} - \frac{64}{125}\)

\(\displaystyle = \frac{16}{125}\)

This answer is not among the choices given.

Example Question #1 : How To Subtract Fractions

Subtract and simplify the fractions:

\(\displaystyle \small \frac{7}{5} - \frac{3}{6}\)

Possible Answers:

\(\displaystyle \small \frac{9}{10}\)

\(\displaystyle \small \small \frac{3}{5}\)

\(\displaystyle \small \small \frac{7}{10}\)

\(\displaystyle \small \small \frac{4}{5}\)

Correct answer:

\(\displaystyle \small \frac{9}{10}\)

Explanation:

First we need to give them common denominators. Remember, whatever you do to the denominator you must do to the numerator:

\(\displaystyle \small \small \frac{6}{6} \times \frac{7}{5} -\frac{5}{5} \times \frac{3}{6}\)

\(\displaystyle \small = \frac{42}{30}- \frac{15}{30}\)

From here, we do algebraic operations to subtract and simplify.

\(\displaystyle \small = \frac{42 - 15}{30}\)

\(\displaystyle \small = \frac{27}{30}\)

\(\displaystyle \small = \frac{9}{10}\)

Example Question #102 : Fractions

Simplify:

  Sat_math_167_03

 

 

Possible Answers:

It is already in simplest terms

ac/bd

ad/bc

a2/c2

a/b/c/d

Correct answer:

ad/bc

Explanation:

Division is the same as multiplying by the reciprocal.  Thus, a/b ÷ c/d = a/b x d/c = ad/bc

 

 

Example Question #1 : How To Divide Fractions

If p is a positive integer, and 4 is the remainder when p-8 is divided by 5, which of the following could be the value of p?

Possible Answers:

17

19

18

20

Correct answer:

17

Explanation:

Remember that if x has a remainder of 4 when divided by 5, x minus 4 must be divisible by 5. We are therefore looking for a number p such that p - 8 - 4 is divisible by 5. The only answer choice that fits this description is 17. 

Example Question #4 : Operations With Fractions

If \dpi{100} \small x=\frac{2}{3}\(\displaystyle \dpi{100} \small x=\frac{2}{3}\) and \dpi{100} \small y= \frac {3}{4}\(\displaystyle \dpi{100} \small y= \frac {3}{4}\), then what is the value of \dpi{100} \small \frac {x}{y}\(\displaystyle \dpi{100} \small \frac {x}{y}\)?

Possible Answers:

\dpi{100} \small 2\(\displaystyle \dpi{100} \small 2\)

\dpi{100} \small \frac{9}{8}\(\displaystyle \dpi{100} \small \frac{9}{8}\)

\dpi{100} \small \frac{8}{9}\(\displaystyle \dpi{100} \small \frac{8}{9}\)

\dpi{100} \small \frac{1}{2}\(\displaystyle \dpi{100} \small \frac{1}{2}\)

\dpi{100} \small \frac{5}{7}\(\displaystyle \dpi{100} \small \frac{5}{7}\)

Correct answer:

\dpi{100} \small \frac{8}{9}\(\displaystyle \dpi{100} \small \frac{8}{9}\)

Explanation:

Dividing by a number (in this case \dpi{100} \small \frac {3}{4}\(\displaystyle \dpi{100} \small \frac {3}{4}\)) is equivalent to multiplying by its reciprocal (in this case \dpi{100} \small \frac {4}{3}\(\displaystyle \dpi{100} \small \frac {4}{3}\)).  Therefore:

\dpi{100} \small \frac {2}{3}\div \frac{3}{4} = \frac{2}{3}\times \frac{4}{3} = \frac{8}{9}\(\displaystyle \dpi{100} \small \frac {2}{3}\div \frac{3}{4} = \frac{2}{3}\times \frac{4}{3} = \frac{8}{9}\)

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