Transmission time $(T_t) = \frac{100}{100 \times 10^3} = 1\; \text{ms}$
$1$ second $\rightarrow 250$ frames
$1$ frame $\rightarrow 1/250$ second or $4\; \text{ms}$
So, $1\;\text{ms} \;\rightarrow 1/4$ frame
Average number of frames generated by the system during one frame transmission time is $G = 1/4.$
Throughput of ALOHA $ = G\times e^{-2G}$
$\qquad = 1/4 \times e^{-1/2} = 0.1516$
Number of frames expected to survive $ = 0.1516 \times 250 = 37.9$ out of $250$ frames.