Explanation:

From the information given for the first two instances,
(9M + 12B) x 4 = (4M + 16B)x 6
36M + 48B = 24M + 96B
36M - 24M = 96B - 48B
12M = 48B
M = 48/12 B = 4B
Now substitute this value for Boys in each instances, let’s calculate the total units of work in terms of Men
(9 M + 12 B) x 4 = (9M + 3 M) x 4 = 48 M . So the total units of work done is 48.
Now getting to the final instance,
6M + 24B = 6M + 24/4M = 12M. That means it is the same as the first case.
So number of days required is 48/12 = 4