THIS WILL CALCULATE: How much to set aside each period if you need so much money in the future?

EXAMPLE: You need to put a new roof on a building in 10 years at an estimated cost of $20,000. How much can I invest every month, so by earning interest at 5% compounded monthly, there will be $20,000 in the bank in 10 years?

Please note:  The result is a negative number because it is a cash outlay.