Question

two taps can fill an empty tank in 20 minutes and 30 minutes respectively. A third pipe can drain a full tank in 5 mins. If a tank is full, and all three pipes are opened together, how long will it take to empty the tank?

Answer 1

20+30=50 minutes to fill it
if it takes 5 minutes to drain it is 50/5 :D
not so bad :)

Answer 2

Hello @rebeccaskell94 sorry but that's the wrong answer.

Answer 3

but thank you for your reply.

Answer 4

D: I"m so sorry :( my bad

Answer 5

Let the tank's capacity be 'x'.
So, in 1 minute, tap A fills x/20 of the tank, and tap B fills x/30th part of it.

Now, both together fill in one minute, (x/20 + x/30)th of the tank = x/12 th of tank

now, tap C (outlet) drains at the rate of x/5 per minute.

So, when all 3 are open simultaneously, the net rate of flow when x/12 in and x/5 out, = x/5 - x/12 = 7x/60 per minute.

So, if the net flow is '7x/60' 'out' per minute, then how much time to completely drain 'x' at this rate?

Answer 6

60/7 minutes

Answer 7

righty-right!

Answer 8

@apoorvk x/5 is negative work since it is draining the water out, so won't we write negative sign with x/5 and +ve sign with x/12?

Answer 9

It's the same thing. if 'outlet rate - inlet rate' is positive, that would mean the outlet has a larger rate, and there is a 'net' outward flow from the tank. If it's negative, it would mean the inlet rate is larger, and there is a net flow 'into' the tank.

vice-versa, for cases when you write 'inlet - outlet'.