Answer
a. 140 beats/minute
b. 160 beats/minute
Work Step by Step
a.
$H=\frac{7}{10}(220-a)$
$ a= 20$
$H=\frac{7}{10}(220-20)=\frac{7}{10}(200)=7\times20=140$
b.
$H=\frac{4}{5}(220-a)$
$ a= 20$
$H=\frac{4}{5}(220-20)=\frac{4}{5}(200)=\frac{800}{5}=160$
