Answer
$20 \leq x \leq 70 \\ y \geq 110-0.5 x \\ y \leq 187-0.85 x$
Work Step by Step
Let us suppose that $x$ is the age and $y$ is the heart rate.
So, the age of the person will be: $20 \leq x \leq 70$
The minimum heart rate at $50 \%$ is $y \geq 0.5( 220-x)$
The maximum heart rate at $50 \%$ is $y \leq 0.85( 220-x)$
So, the system of inequalities is:
$20 \leq x \leq 70 \\ y \geq 110-0.5 x \\ y \leq 187-0.85 x$
