SSC CGL 2024 Tier-I · 18 September 2024, Shift 2 · Quantitative Aptitude · Question 13 of 99

Text as printed on a candidate's response sheet for this shift; answer per SSC's tentative key as marked on that sheet. Solution last reviewed 10 September 2026.

Where this paper and its answers come from

SSC does not publish CGL question papers or a public answer key. The questions and options here are reproduced as printed on a candidate's own response sheet for this shift — the Commission's text, unchanged. Each answer is SSC's tentative key as marked on that sheet; SSC allowed challenges to it and never publishes a final key. The solutions are written by UnlockIAS.

Pipes A and B can fill a tank in 6 hours and 8 hours, respectively. Both pipes are opened together for 3 hours. After that pipe A is closed, and B continues to fill the tank. In how many hours will the tank be filled?

(a)4
(b)3
(c)6
(d)2

Answer

Why

Correct — A. Take the tank as 24 units, the LCM of 6 and 8, so no fractions appear.

A fills 24 ÷ 6 = 4 units per hour
B fills 24 ÷ 8 = 3 units per hour
Together they fill 7 units per hour

In the first 3 hours: 7 × 3 = 21 units, leaving 24 − 21 = 3 units.

B alone clears those at 3 units per hour, so it needs 1 hour further.

The question asks when the tank is full, counted from the start: 3 + 1 = 4 hours → option (a).

Why the others are wrong

(b)
3 — 3 hours is the figure handed to you in the stem, not the answer. At the 3-hour mark the tank holds 21 of its 24 units, which is seven-eighths full.
(c)
6 — 6 hours is pipe A's own solo time, lifted straight from the first sentence. With B running alongside for three hours the tank is full well before that.
(d)
2 — 2 hours is neither the extra hour B works after A closes nor the 4-hour total. No step of the working produces it.

Concept

Pipe-and-cistern questions are work questions in disguise. Take the tank as the LCM of the stated times, turn each pipe into units per hour, and add the rates.

At 24 units A becomes a 4-unit pipe and B a 3-unit pipe, and every number in the solution stays whole.

The thing to settle before dividing is what the final sentence counts. In how many hours will the tank be filled counts from the moment the pipes were first opened, so the three shared hours are part of the answer.

Read the last sentence closely. 17 Sep 2024, 09:00, Quant Q.17 asks after how many hours the remaining part of the tank will be filled, which wants only the leftover stretch. This row asks for the whole fill, so the shared hours count.

Key facts

Taking the tank as the LCM of the individual times makes A a 4-unit-per-hour pipe and B a 3-unit-per-hour pipe out of 24.
In 3 hours the two together deliver 21 of the 24 units, leaving one-eighth of the tank.
B alone clears the last 3 units in exactly 1 hour, so the tank is full 4 hours after the start.

Study next

Pipes and cisterns solved by the LCM-of-times unit methodFilling problems where one pipe is withdrawn part-way throughLeaks and outlet pipes treated as negative rates

Common traps

Answering 1, the stretch B works alone, when the question asks for the total elapsed time.
Adding the times as 6 + 8, or averaging them, instead of adding the rates.
Losing the remainder: after 3 hours one-eighth of the tank is left, not one-quarter.

SSC varies this skeleton by changing what happens part-way through the fill. 17 Sep 2024, 09:00, Quant Q.17 adds an emptying pipe C and shuts A after five hours; 23 Sep 2024, 16:00, Quant Q.11 keeps both pipes open but starts a leak once the tank is one-third full.

Related PYQs

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