A sum of money at simple interest doubles in 7 yrs. It will become 4 times in:

21 years

Let's assume sum of money equal to 100, after 7 years it becomes 200. That means Simple interest in 7 years is 100. 

For sum to be four times the Simple interest needs to be 300

1st 7 years - 100 Simple interest

Next 7 years - 100 Simple interest

Next 7 years - 100 Simple interest

Thus, in the next 21 years the simple interest will be 300 or we can say in the next 21 years the amount will be 400 or 4 times