Simple Interest - General Questions

A sum of money amounts to Rs. 9,800 in 4 years and to Rs. 11,200 in 6 years at simple interest. What is the sum?

A sum of Rs. 18,000 earns a simple interest of Rs. 10,800 in 6 years. What is the rate of interest per annum?

A sum of Rs. 12,000 earns a simple interest of Rs. 3,840 in 4 years. What is the rate of interest per annum?

At 5% per annum simple interest, what principal yields Rs. 400 as interest in 2 years?

In how many years will a sum of money double itself at 4% per annum simple interest?