Seven copies of a book can be bought for the certain sum payable at the end of a year and eight copies of the same book can be bought for the same sum in ready money; what is the rate percentage (correct up to two decimal places) of interest?

This is equivalent to: 7 copies at year-end price (with interest) = 8 copies at ready money price. Let rate = r%. Then 7(1 + r/100) = 8, giving 1 + r/100 = 8/7, so r = 100/7 = 14.29%. This relationship comes from the fact that with simple interest, the future value equals present value times (1 + rate). Option B is correct.