M men agree to purchase a gift for Taka D. If three men drop out how much more will each have to contribute towards the purchase of the gift?

At First,

M men contributed to Tk. D

1 man contributed to Tk. D / M

After 3 men leaving,

(M - 3) contributed to Tk. D

so, 1 man contributed to Tk. D / (M - 3)

After 3 men leaving, the individual amount for each men increased. So, D / (M - 3) > D/M

Now,

Diference = {D / (M-3)} - (D/M)

= (DM - DM + 3D) / M * (M - 3)

= 3D / (M2 - 3M)