In a particular time, a sum becomes 4 times of itself on simple interest at the rate of 10% per annum. What is the rate of interest if the same sum becomes 7 times of itself in the same duration?

20%

                Principal   :  Amount   :    S.I.   ⇒     Rate

4 times ⇒        1       :        4       :     3     ⇒      10%

                                                         ⇓ ×2          ⇓ ×2

7 times ⇒        1       :        7       :     6     ⇒      20%