In a particular time, a sum becomes 3 times of itself on simple interest at the rate of 10% per annum. What is the rate of interest if the same sum becomes 5 times of itself in the same duration?

20%

                Principal   :  Amount   :    S.I.   ⇒     Rate

3 times ⇒        1       :        3       :     2     ⇒      10%

                                                         ⇓ ×2          ⇓ ×2

5 times ⇒        1       :        5       :     4     ⇒      20%