Goods were bought for $Rs. 600$ and sold the same for $Rs. 688.50$ at a credit of $9$ months and thus gaining $2$%. The rate of interest per annum is:

Given a 2 percent gain, the selling price of Rs. 688.50 at 9 months credit has a present value of 688.50 divided by 1.02, which equals Rs. 675. This means the cash price is Rs. 675, yielding a total interest of 675 minus 600, equaling Rs. 75 on the principal of Rs. 600 for 9 months. Using the simple interest formula, the rate is calculated as (75 multiplied by 100) divided by (600 multiplied by 0.75), which equals 16 and 2/3 percent. The result is 16 and 2/3 percent.

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