A sum of money becomes 7/6 of itself in 3 years at a certain rate of simple interest. The rate per annum is?

5 5/9 %

Let sum = x. Then, amount = 7x/6
S.I. = 7x/6 - x = x/6; Time = 3 years.
Rate = (100 * x) / (x * 6 * 3) = 5 5/9