If the simple interest on a certain sum of money for 2 years in one fifth of the Principal, then find rate of interest per annum. 

A certain sum doubles in 7 years at simple interest. The same sum under the same interest rate will become 4 times in how many years.:

A person invested some amount at the rate of 12 % simple interest and a certain amount at the rate of 10 % simple interest for 1 year. He received yearly interest of Rs. ₹ 130, but if he had interchanged the amount invested, he would have received 4 more as interest. How much did he invest at 12 % simple interest? 

A sum of Rs.1600 gives a simple interest of Rs.252 in 2 years and 3 months. The rate of interest per annum is 

A sum of ₹ 3000 yields an interest of ₹ 1080 at 12% per annum simple interest in how many years ?

The simple interest on an amount at the rate of 13.5 p.c.p.a. for 10 years is Rs. 765.45. What is the sum?

A certain sum of Rs.4200 becomes Rs5523 at 11% p.a simple interest in a certain time. What is the simple interest on the sum of Rs. 4800 at the same rate and in same time period?

In what time will the annual simple interest rate at $$ {6{1\over4}{\%}}$$ , 72 rupees becomes 81 rupees?