At what interest rate per annum will a sum of money double itself in

The question clearly needs to specify that it's talking purely about simple interest, since I think most people reading it would assume the interest compounds annually. If it doubles in 8 years, and we are adding the same amount every year (which is what happens with simple interest), then we need to add 1/8 of the investment each year, or 12.5%.

If the interest did compound annually, then it's impossible to answer precisely using GMAT methods. There is an estimate, used in financial math, called the "rule of 72" that says it should take 72/x years for money to double, where x is the percent interest rate, so, since 72/9 = 8, the answer in that situation would be *approximately* 9%, but that's not an exact value (the exact answer is very close to 9.05%). But the GMAT would never test the 'rule of 72' because it's not going to be familiar to all test takers.