10. 79.

Rule of 72 [LO 6.4] Earlier, we discussed the Rule of 72, a useful approximation for many interest rates and periods for the time it takes a lump sum to double in value. For a 10 per cent interest rate, show that the ‘Rule of 73' is slightly better. For what rate is the Rule of 72 exact? (Hint: Use the Solver function in Excel.)