Answer
$a. \displaystyle \quad H=\frac{9}{10}(220-a)$
$b. \quad 162$ 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
$H=\displaystyle \frac{9}{10}(220-a)$
$b.$
Substitute 40 for a
$H=\displaystyle \frac{9}{10}(220-40)=\frac{9\cdot 180}{10}$
... reduce the fraction with 10
$= \displaystyle \frac{9\cdot 18}{1}$
$=162$ beats per minute.
