A sink can be filled by a pipe in 2 minutes but it takes 4 minutes to drain a full sink. if both the pipe and the drain are open, how long will it take to fill the sink?

Answer 1

1/2 - 1/4 = 1/x
x = 4

Check:
water comes in at 1/2 sink/minute
water drains out at 1/4 sink/minute
net flow: 1/2-1/4 = 1/4 sink/min
So, it takes 4 minutes to fill the sink