He should invest $80,000 if he wants $8,000 at 10% interest. He can withdraw $8,000 at the end of each year. This is because if $8,000 is 10% of what he has in his bank account, to get the full amount (100%), we can multiply by 10, because 10% x 10= 100%. So $8,000 x 10= $80,000.
