A sum of money invested at simple interest becomes 17/10 of itself in 2 years and 6 months. What is the rate of interest per annum?

28 percent

If Principal (P) = 100

Then, Interest(I) = Rate(R) × Time(T)   ....  (i)

So,

Amount = \(\frac{17}{10}\) × 100 = 170

Interest = Amount - Principla = 170 - 100 = 70

T = 2\(\frac{1}{2}\) years OR \(\frac{5}{2}\)

From (i)

⇒ 70 = R×\(\frac{5}{2}\)

⇒ R=28%