Answer:
$11,508.25
Step-by-step explanation:
your son will start college in 14 years, and the present value of his college tuition = $40,000 x 3.4651 (PVIFA, 6%, 4 periods) = $138,604
your daughter will start college in 17 years, so you need in today's dollars $138,604
you will need to save enough money to cover both tuitions;
money required to cover your son's tuition = $138,604 / 21.015 (FVIFA, 6%, 14 periods) = $6,595.48
money required to cover your daughter's tuition = $138,604 / 28.213 (FVIFA, 6%, 14 periods) = $4,912.77
total annual savings = $11,508.25