The effective distance travelled by drunkard in 8 steps = 5-3 = 2m
Therefore, he takes 32 steps to move 8 m.
Now, he will have to cover 5 m more to reach the pit , for which he has to take only 5 forward steps.
Therefore, he will have to take = 32 + 5 = 37 steps to move 13 m. Thus, he will fall into the pit after taking 37 steps i.e. after 37 s from the start.