In a particular time, a sum becomes 5 times of itself on simple interest at the rate of 15% per annum. What is the rate of interest if the same sum becomes 9 times of itself in the same duration?

30%

                Principal   :  Amount   :    S.I.   ⇒     Rate

5 times ⇒        1       :        5       :     4     ⇒      15%

                                                         ⇓ ×2          ⇓ ×2

9 times ⇒        1       :        9       :     8     ⇒      30%