Let x be the number of people in the beginning.
Amount deposited for 6 months = 6*x*x = 6x^2
Number of people for the next 3 months = x -x/4 = 3x/4
Amount deposited for the next 3 months = 3 * (3x/4)^2
Number of people for the last 3 months = 3x/4 - (1/3 * 3x/4) = x/2
Amount deposited for the last 3 months = 3* (x/2)^2
Total amount = 6x^2 + 27x^2/16 + 3x^2/4 = 4860
x= 24 (option B)
I'm telling this because you don't get it. You think you get it which is not the same as actually getting it. Get it?