SSC CGL 2024 Tier-I · 17 September 2024, Shift 1 · Quantitative Aptitude · Question 17 of 98

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 6 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 15 hours and 25 hours, respectively, whereas pipe C can empty the full tank in 40 hours. All three pipes are opened together, but pipe A is closed after 5 hours. After how many hours will the remaining part of the tank be filled?

Answer

Why

Correct — B. Take the tank as 600 units, the LCM of 15, 25 and 40.

A fills 600⁄15 = 40 units/h, B fills 600⁄25 = 24 units/h, C drains 600⁄40 = 15 units/h.

All three open: 40 + 24 − 15 = 49 units/h, so 5 hours put in 49 × 5 = 245 units.
Remaining = 600 − 245 = 355 units.

A is now closed, leaving B and C at 24 − 15 = 9 units/h.
355 ÷ 9 = 39 4⁄9 hours → option (b), the picture reading 39 4⁄9.

Why the others are wrong

(a)
Option (a) shows 44 4⁄9, which is the total time from the start: the 5 hours with all three pipes plus 39 4⁄9 more. The stem asks only for the remaining part.
(c)
Option (c) shows 41 4⁄9 = 373⁄9, which needs 373 units still to fill. The first five hours deliver 245 of the 600, so 355 remain.
(d)
Option (d) shows 43 4⁄9 = 391⁄9, i.e. 391 units left, which credits the opening five hours with only 209 units. Recheck the net rate: 49 × 5 = 245.

Concept

Pipe questions become plain arithmetic once you stop working in fractions of a tank. Take the capacity as the LCM of the given times — LCM(15, 25, 40) = 600 — and every rate turns into a whole number.

An outlet pipe carries a negative rate, so it is subtracted, not added: the net with all three running is 40 + 24 − 15 = 49 units/h.

The tank then has two phases. Five hours at the three-pipe rate, then the rest at the two-pipe rate that survives A's closing. Track the units filled, not the time.

All four options are pictures of a mixed fraction sharing the same 4⁄9, so the fraction cannot separate them — only the whole-number part is in dispute, and that comes from the 355 units left after five hours.

Key facts

Taking the tank as the LCM of the fill times makes every rate a whole number: 600 gives 40, 24 and 15 units per hour.
An emptying pipe subtracts from the net rate, so all three together fill 49 units per hour.
After 5 hours the tank holds 245 of its 600 units, leaving 355 for B and C at 9 units per hour.

Study next

Pipes opened in rotation, one hour each in turnA leak that lengthens a known filling time, handled as a negative rateWork-and-wages problems, the same rate arithmetic with people in place of pipes

Common traps

Answering with the total time from the start, 44 4⁄9 hours.
Giving the outlet pipe a positive rate, which makes the net 79 units per hour.
Running the second phase at B's rate alone and forgetting that C keeps draining.

SSC's standard build is two inlets, one outlet, and a pipe closed or opened part-way through, with the ask pinned to the leftover part rather than the whole job.

The three-pipe net rate is asked 17 Sep 2024, 16:00, Quant Q.9 (12 h and 18 h inlets against an 8-hour outlet). A mid-run change of pipes is asked 9 Sep 2024, 12:30, Quant Q.25, where one pipe is shut and another opened after 10 minutes.

Related PYQs

No directly related past PYQ was found.