Answer : Option B

Explanation :

Pervez can complete the work in 20 days
Sunny can complete the same job in 15 days
Let the total work = LCM of both
Therefore, the LCM of 20 and 15 is 60, that means total work = 60 units
Now, Pervez's one-day work efficiency = 60/20 = 3 units
Sunny's one-day work efficiency = 60/15 = 4 units
ATQ, (Pervez+Sunny) together works for 6 days
That means (Pervez+Sunny) have done 7*6 = 42 units work in 6 days
Now, the remaining work will be 60-42 = 18 units, and it will finish in 4 days.
Now, Sunny is replaced with Ashu:
So, per days work required to be finished = 18 units / 4 days = 4.5 units
But, we know that Pervez's one-day work = 3 units, so Ashu's one-day work = 4.5-3 = 1.5 units
Therefore, Ashu alone can finish the total work in 60 units /1.5 days = 40 days