lets assume total tank capacity = 60 units
each of the pipes working alone can finish theor tasks in following hours:

A ---------------- B -------------- C ------------ D
10 hrs --------- 15 hrs --------- 20 hrs ------- 30 hrs
6 units/hr ----- 4 units/hr ---- 3 units/hr ---- 2 units/hr

We know that pool got filled in 12 hrs therefore net units flowing into the pool per hour has to be 60/12 = 5 units

if pipes A and B are filling the pool they fill 10 units / hr
Pipes C and D are draining pools they drain out 5 units / hr
hence net pool filled per hour = 5 units

that should fill the pool in 12 hours

hence C and D are outlet pipes

Ans B

Aman verma wrote:Q: A swimming pool is connected with four pipes A,B,C,D of which two can fill the pool and the other two can empty it. The time taken by A,B,C,D to finish their jobs are 10 hr, 15 hr, 20 hr and 30 hr respectively. All four pipes are opened when the pool was empty. It took 12 hrs to fill it completely. Which two are the outlet pipes ?

a) A & B

b) c & D

c) A & C

d) B & D

e) B & C

Let the pool = 60 units.

Rate for A = w/t = 60/10 = 6 units per hour.
Rate for B = w/t = 60/15 = 4 units per hour.
Rate for C = w/t = 60/20 = 3 units per hour.
Rate for D = w/t = 60/30 = 2 units per hour.

Two of these rates must serve to FILL the pool, implying two POSITIVE RATES.
The other two rates must serve to EMPTY the pool, implying two NEGATIVE RATES.
When the four rates are combined, the time to fill the pool must be 12 hours, implying the following COMBINED RATE:
w/t = 60/12 = 5 units per hour.
Only one combination works:
A + B - C - D = 6 + 4 - 3 - 2 = 5 units per hour.
Thus, A and B must be filling the pool, while C and D serve as OUTLET pipes emptying the pool.

The correct answer is B.