We assume the retirement account is intended to pay out $5000 per month for 25 years. The amortization formula can be used to find the required amount. The monthly payment A based on principal P with interest at annual rate r for t years satisfies the relation ...
First divide 3 month cost by 3-$89.85/3= $29.95. Do the same for 8 months( $239.60/8= $29.95. Since $29.95 is common for both take that $29.95 x 14 months equaling $419.30.