A sum of money becomes five times at 8% simple rate of interest per annum. At what rate percent will it become seven fold?

12%

Let principal be P and  time be T,

Amount = P(1 + \(\frac{8T}{100}\))

⇒ 5P = P(1 + \(\frac{8T}{100}\))

⇒ 5 = 1 + \(\frac{8T}{100}\)

⇒ \(\frac{8T}{100}\) = 4

⇒ T = 50

Now, 

⇒ 7P = P(1 + \(\frac{RX50}{100}\))

⇒ 7 = 1 + \(\frac{50R}{100}\)

⇒ R = 12%