$\large S_{slotted} = G \times e^{-G}$
$\large G = N \times fps \times T_{fr}$
$\text{frame size} = 20 \ \text{bit} $
Here number of stations are not given and frames generated by whole system is given which is $1000 fps$
so, $G = 1000 \ \times \frac{20}{ 20 \times 10^3} $
$G = 1$
$\large S_{slotted} = 1 \times e^{-1} = 0.368 = 36.8 \text{%}$