If n is number of trails, p is probability of success and q is probability of unsuccess then, Mean =np and variance =npq. Here np+npq=24‌‌...(i) np.npq=128‌‌... (ii) and q=1−p‌‌...(iii) from eq. (i), (ii) and (iii) : p=q=‌
1
2
and n=32. ∴ Required probability =p(X=1)+p(X=2) =‌32C1⋅(‌