A person borrowed Rs `10000` from four men. If he had to repay the first man for his Rs `2000` at the rate of simple interest of `8%` per annum, the second man for his Rs `4000` at the rate of simple interest of `7(1)/(2)%` per annum and the third man for his Rs `1400` at a rate of simple interest of `8(1)/(2)%` per annum, then in what rate of simple interest in percent per annum, the fourth man should be paid for his rest part of principal so as to the average rate of simple interest be `8.13%` per annum?