At what rate percent compound interest per annum will Rs. 640 amount to Rs. 774.40 in 2 years?

The difference between the S.I. and C.I. on a certain sum of money for 2 years at 4$ per annum is Rs. 20. Find the sum.

In what time will Rs. 1000 amount to Rs. 1331 at 10% per annum, compound interest?

Find the rate percent per annum if Rs. 2000 amount to Rs. 2662 in 1years, interest being compounded half-yearly?

Kamala borrowed from Ratan a certain sum at a certain rate for two years simple interest. She lent this sum at the same rate to Harti for two years compound interest. At the end of two years she received Rs. 210 as compound interest, but paid Rs. 200 only as simple interest. Find the sum and the rate of interest.