A watering can holds 4.2 liters if a small cactus requires 310 ml of water once every 3 weeks. How many of the same cactus plants can be watered at once without refilling the watering can?

Thirteen cactus plants can be watered once with a 4.2 liter capacity watering can. There are 4,200 milliliters in 4.2 liters. Thirteen plants use up 4,030 milliliters of water from the can. That leaves 170 milliliters, which won't meet the watering needs of cactus plant number 14. So it's at cactus plant number 14 that the gardener needs to refill the watering can.