# Chapter P - Section P.1 - Algebraic Expressions, Mathematical Models, and Real Numbers - Exercise Set - Page 18: 129

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. $H=\frac{4}{5}(220-a)$ $a= 20$ $H=\frac{4}{5}(220-20)=\frac{4}{5}(200)=\frac{800}{5}=160$

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