Pipes A, B and C can fill a tank in 20 hrs, 30 hrs and 40 hrs respectively. Pipes A, B and C are opened 6 AM, 7 AM and 8 AM respectively on the same day. At what time the tank will be filled?

4 PM

Let total capacity of tank = 120 ltr.

Efficiency of A = \(\frac{120}{20}\) = 6 ltr/hr.

Efficiency of B = \(\frac{120}{30}\) = 4 ltr/hr.

Efficiency of C = \(\frac{120}{40}\) = 3 ltr/hr.

Now,

A's work of 2 hrs (6am to 8am) = 6 × 2 = 12 ltr.

B's work of 1 hrs (7am to 8am) = 4 × 1 = 4 ltr.

Total tank filed till 8 am =12 + 4 = 16 ltr

Remaining = 120 - 16 = 104

Remaining tank will be filled by all three pipes in = \(\frac{104}{6+4+3}\) = 8hrs.

So, tank will be filled at:- 8 a.m. + 8 hrs. = 4 p.m.