David Snow and Robert Snow are twins, aged 20. Advanced for their age in some respects, they were both working and also planning their financial future. Both felt they could commit $5,000 per year for their retirement expected at age 65. However, David Snow planned to start right away on his commitment by investing $5,000 per year in his tax-free savings account for the next 15 years and then make no further commitment to this account until he reached retirement at age 65 (thirty years). She would leave her money in the tax-free account, accumulating interest during this 30-year period. Robert Snow, on the other hand, was going to spend freely for the next 15 years and then make his commitment of $5,000 per year to the tax-free savings account for the following 30 years, until her retirement at age 65.

Required: (a) How much would David and Robert have in their tax-free retirement account if interest rates are 10% throughout their lifetime? (b) How much would David and Robert have in their tax-free retirement account if interest rates are 3% throughout their lifetime?