Answer
$$71\frac{37}{40} \ gallons$$
Work Step by Step
We first approximate the gallons of gas used as whole numbers to find an estimate for the number of gallons used:
$$ =15+20+20+18 \\ = 73 \ gallons $$
The exact answer is:
$$ 14\frac{1}{2} + 19\frac{3}{8} + 20\frac{3}{10} + 17\frac{3}{4}
\\ = 70 +(\frac{20}{40} + \frac{15}{40} + \frac{12}{40} + \frac{30}{40}) \\ =70\frac{77}{40} \\=71\frac{37}{40} \ gallons$$
