Answer
$\dfrac{1}{3}$ or, $33.3 \%$
Work Step by Step
The probability of a missed call can be computed as the ratio of the length corresponding to when the phone is ON to the total length of time.
$P(\text{Missed call})=\dfrac{20-0}{60-0}\\=\dfrac{20}{60}\\=\dfrac{1}{3}$
or, $33.3 \%$
