Ben is reheating leftover pizza in the oven. The pizza (right before he places it in the oven) is in a refrigerator set at 40 deg F. Ben is patient and waits to put the pizza in the oven until the oven reaches 400 deg F. If the oven heats up at a rate of 20 deg F per minute, how many minutes does Ben have to wait before putting the pizza in the oven?

Ben must wait for 18 minutes before putting his pizza in the oven, as it takes the oven 18 minutes to go from 40 degrees F to the required 400 degrees F at a rate of 20 degrees F per minute.

Ben needs to calculate how long he must wait for the oven to reach the desired temperature for reheating his pizza. The oven is heating up at a rate of 20 degrees F per minute, and it needs to go from room temperature to 400 degrees F. At an initial refrigerator temperature of 40 degrees F, the oven needs to increase by 400 - 40 = 360 degrees F. Dividing the total temperature increase needed by the rate of temperature increase per minute gives us the total time Ben must wait:

Time (minutes) = Total increase in temperature (degrees F) / Rate of temperature increase (degrees F per minute)

Time (minutes) = 360 / 20

Time (minutes) = 18 minutes

Therefore, Ben must wait 18 minutes before putting the pizza in the oven.