A sum of money doubles itself in 7 years at simple interest. In how much time will it become 5 times of itself?

28 years

Let Principal (P) = 1R

ATQ, 

                 Principal     :    Amount   :    S.I       :      Time

i) Double:         1         :         2       :      1        :       7 years

                                                            ⇓×4             ⇓×4

ii) 5 times:       1          :         5      :       4        :      28 years