A sum becomes 7/5 times of itself at simple interest of 8% p.a. in:

Let principal be P and  time be T,

Amount = P(1 + \(\frac{8T}{100}\))

⇒ \(\frac{7}{5}\)P = P(1 + \(\frac{8T}{100}\))

⇒ \(\frac{7}{5}\) = 1 + \(\frac{8T}{100}\)

⇒ T = 5 years