Simple Interest - General Questions

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?

In how many years will a sum of money double itself at 20% per annum simple interest?

A sum of money amounts to Rs. 12,040 in 4 years and to Rs. 12,900 in 5 years at simple interest. What is the sum?

A sum of Rs. 6,000 earns a simple interest of Rs. 600 in 2 years. What is the rate of interest per annum?

In how many years will a sum of money double itself at 12.5% per annum simple interest?