This question requires you to subtract fractions as well as convert hours to minutes.  

Subtracting $\displaystyle 3\frac{3}{4}$ hours $\displaystyle \left ( \frac{15}{4} \right )$ from $\displaystyle 7\frac{1}{2}$ hours $\displaystyle \left ( \frac{15}{2}\ or\ \frac{30}{4} \right )$

you get $\displaystyle 3\frac{3}{4}$ hours $\displaystyle \left ( \frac{15}{4} \right )$.  

3 hours is 180 minutes $\displaystyle \left ( 3\cdot 60 \right )$

and $\displaystyle \frac{3}{4}$ of an hour is 45 minutes $\displaystyle \left ( \frac{3}{4}\cdot 60 \right )$.

Thus the answer is

$\displaystyle 180+45=225\ minutes$

"Borrow" 1 from the 9 to form $\displaystyle 8 \frac{7}{7}$. You can then subtract integers and fractions vertically:

$\displaystyle 8 \frac{7}{7}$

$\displaystyle \underline{5 \frac{3}{7}}$

$\displaystyle 3 \frac{4}{7}$

Rewrite as the difference of improper fractions:

$\displaystyle 6 \frac{1}{5} - 4 \frac{1}{2} = \frac{6 \times 5 + 1}{5} - \frac{4 \times 2 + 1}{2} = \frac{31}{5} - \frac{9}{2}$

Rewrite with a common denominator, then subtract numerators:

$\displaystyle \frac{31}{5} - \frac{9}{2} = \frac{2 \times 31}{2 \times 5} - \frac{9\times 5}{2\times 5} = \frac{62}{10} - \frac{45}{10} = \frac{17}{10}$

Rewrite as a mixed number:

$\displaystyle 17 \div 10 = 1 \textrm{ R } 7$

$\displaystyle \frac{17}{10} = 1 \frac{7}{10}$

Rewrite as the difference of improper fractions:

$\displaystyle 8 \frac{4}{5} - 4 \frac{1}{3} = \frac{8 \times 5 + 4}{5} - \frac{4 \times 3 + 1}{3} = \frac{44}{5} - \frac{13}{3}$

Rewrite with a common denominator, then subtract numerators:

$\displaystyle \frac{44}{5} - \frac{13}{3} = \frac{44\times 3}{5 \times 3} - \frac{5 \times13}{5 \times3} = \frac{132}{15 } - \frac{65}{15 } = \frac{67}{15 }$

Rewrite as a mixed number:

$\displaystyle 67\div 15 = 4 \textrm{ R } 7$

so

$\displaystyle \frac{67}{15 } = 4 \frac{7}{15 }$

"Borrow" 1 from the 5 to form $\displaystyle 4 \frac{5}{5}$. You can then subtract integers and fractions vertically:

$\displaystyle 4 \frac{5}{5}$

$\displaystyle \underline{1 \frac{4}{5}}$

$\displaystyle 3 \frac{1}{5}$

$\displaystyle \frac{17}{40} - \frac{1}{40} + \frac{19}{40}$

$\displaystyle =\frac{17-1+19}{40}$

$\displaystyle =\frac{16+19}{40}$

$\displaystyle =\frac{35}{40} = \frac{35\div 5}{40\div 5} = \frac{7}{8}$

By order of operations, subtractions and additions are carried out in left-to-right order, so subtract 1.73 from 7.89 first. This is best done vertically, aligning decimal points:

$\displaystyle 7.89$

$\displaystyle \underline{1.73}$

$\displaystyle 6.16$

Now add 2.50 to the difference (note that a zero has been added to the end), again aligning vertically by decimal point:

$\displaystyle 6.16$

$\displaystyle \underline{2.50}$

$\displaystyle 8.66$

Subtract vertically by aligning the decimal points, making sure you append the 8 with a decimal point and two placeholder zeroes:

$\displaystyle 8.00$

$\displaystyle \underline{3.27}$

$\displaystyle 4.73$

Rewrite vertically, lining up the decimal digits. Subtract as you would two integers. (Note that you are appending zeroes to the 19.)

$\displaystyle \begin{matrix} 19.000 \\ - \underline{0.006}\\ 18.994 \end{matrix}$

Rewrite the first fraction in eighths, as $\displaystyle LCD (2,8) = 8$:

$\displaystyle 7\frac{1}{2} = 7\frac{1 \times 4}{2 \times 4} = 7\frac{4}{8}$

Write vertically:

   $\displaystyle 7\frac{4}{8}$

$\displaystyle -\underline{3\frac{7}{8}}$

Now "borrow" one from 7 and add it to the $\displaystyle \frac{4}{8}$, then subtract integer and fractional parts separately:

  $\displaystyle 6\frac{12}{8}$

$\displaystyle -\underline{3\frac{7}{8}}$

   $\displaystyle 3\frac{5}{8}$