In what time will Rs. 6250 amount to Rs. 6632.55 at 4% compound interest payable half yearly?

Since Interest accumulates half yearly so effective rate = R/2 = 4/2 = 2.
And effective time is 2t.
∵ 6250 [1 + 2/100]^2t = 6632.55
⇒ (1 +2/100)^2t = 663255/625000 = 132651/125000 = (51/50)³
⇒ (51/50)^2t = (51/50)³
⇒ 2t =3
∴ t = 3/2 years