Question

Two pipes M and N can fill a tank in 20 minutes and 30 minutes respectively. These pipes are opened alternately for 1 minutes each and pipes N is opened first, in how many minutes, the tank fill-in?

1. 20 min
2. 15 min
3. 22 min
4. 24 min
5. None of these

Answer 1

Correct Answer - Option 4 : 24 min

Given:

Time is taken by pipe M = 20 minutes

Time is taken by pipe N = 30 minutes

Calculation:

Let the total capacity of tank be LCM (20, 30) = 60 litres

Now,

Tank filled by M in 1 minute = 60/20

⇒ 3 litre/min

Tank filled by N in 1 minute = 60/30

⇒ 2 litre/min

Tank filled by (M + N) in 1 minute = (3 + 2)

⇒ 5 litre/min

Now,

According to the question,

For first minute pipe N is opened = 2 litre

For second minute pipe M is opened = 3 litre

For third minute pipe N is opened = 2 litre

For fourth minute pipe M is opened = 3 litre

Similarly, So…. On

Now,

We conclude that –

2 minutes = 5 litre

× 12                × 12

24 min   =      60 litre

∴ The tank will be filled in 24 minutes.