A hot - water faucet can fill a bathtub in 20 minutes; and the cold-water faucet can fill it in 10 minutes. How long will it take to fill the same bathtub with both faucets running?
It every minute the tub is filled 1/20 by the hot faucet, and 1/10 per minute by the cold faucet. every minute they both fill 3/20th of the tub. in 6 minutes the tub will be filled 18/20ths, and so in 6 2/3 minutes the tub will be fully filled.