#### Answer

from $80$ to $110$ minutes

#### Work Step by Step

Use the given information in the problem to create an inequality based on the original equation. Then subtract $20$, divide by $0.40$, and add $60$ on all three sides to solve for $x$.
$C=20+0.40(x-60)$
$28\leq C\leq40$
$28\leq20+0.40(x-60)\leq40$
$8\leq0.40(x-60)\leq20$
$20\leq(x-60)\leq50$
$80\leq x\leq110$