A sink can be filled from the faucet in 5 minutes. It takes only 3 minutes to empty the sink when the drain is open. If the sink is full and both the faucet and the drain are open, how long will it take to empty the sink??
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simple way
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assume the sink can hold 15 units of water [ LCM of 5 & 3 ]
rate at which faucet fills = 15/5 = 3/min
rate at which drain empties = 15/3 = 5/min
net rate at which sink empties when both open = 2/min
time needed to empty full sink = 15/2 = 7.5 mins <--------
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assume the sink can hold 15 units of water [ LCM of 5 & 3 ]
rate at which faucet fills = 15/5 = 3/min
rate at which drain empties = 15/3 = 5/min
net rate at which sink empties when both open = 2/min
time needed to empty full sink = 15/2 = 7.5 mins <--------