Answer
$a. \displaystyle \quad H=\frac{7}{8}(220-a)$
$b. \quad 175$ beats per minute.
Work Step by Step
$a.$
Based on the form of the formulas in previous exercises, we will
subtract the age from 220, and
multiply with the wanted factor of heart rate
$H=\displaystyle \frac{7}{8}(220-a)$
$b.$
Substitute 20 for a
$H=\displaystyle \frac{7}{8}(220-20)=\frac{7\cdot 200}{8}$
... reduce the fraction with $8$
$= \displaystyle \frac{7\cdot 25}{1}$
$=175$ beats per minute.
