SSC CGL 2025 Tier-I · 17 September 2025, Shift 1 · Quantitative Aptitude · Question 5 of 100

We transcribed this text from the pictures on a candidate's response sheet for this shift; the answer is the key SSC colour-marked on that sheet. Solution last reviewed 13 September 2026.

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Pipe A is capable of filling a tank in 16 minutes while Pipe B can fill the same tank in 24 minutes. Both pipes are opened together, but after 4 minutes, the rate of Pipe A doubles and the rate of Pipe B becomes half of its original. How many more minutes will it take to fill the tank completely?

(a)4 min
(b)5 min
(c)6 min
(d)7 min

Answer

Why

Correct — A. Take the tank as the LCM of 16 and 24, 48 units.

Pipe A: 48 ÷ 16 = 3 units⁄min
Pipe B: 48 ÷ 24 = 2 units⁄min
First 4 minutes together: 4 × (3 + 2) = 20 units

Left to fill: 48 − 20 = 28 units
New rates: A doubles to 6, B halves to 1, so 7 units⁄min
Time = 28 ÷ 7 = 4 minutes → option (a)

Why the others are wrong

(b)
5 min — 5 min at the new 7 units⁄min would fill 35 units, more than the 28 left. Even at the old 5 units⁄min the remainder takes 5.6 min, not 5.
(c)
6 min — 6 min at 7 units⁄min fills 42 units, far past the 28 still empty. The tank is already full after 4 of those minutes.
(d)
7 min — 7 min is what you get by halving B but leaving A at 3 units⁄min: 28 ÷ (3 + 1) = 7. Pipe A's rate doubles to 6, so the combined rate is 7.

Concept

Work in units, not fractions. Taking the tank as the LCM of the filling times, 48, makes every rate a whole number: A fills 3 units a minute, B fills 2.

When the rates change partway, split the job into two phases. Phase one runs at the old rates for the given time. Phase two divides whatever is left by the new combined rate.

The question asks for the extra minutes after the change, so the first 4 minutes are not added back.

Check in fractions: together the pipes fill 5⁄48 a minute, so 20⁄48 in 4 minutes. The new rate is 1⁄8 + 1⁄48 = 7⁄48 a minute, and 28⁄48 ÷ 7⁄48 = 4.

Key facts

A pipe that fills a tank in t minutes fills 1⁄t of it each minute.
Taking capacity as the LCM of the filling times turns rates into whole units.
Doubling a pipe's rate halves the time it would take alone: 16 minutes becomes 8.

Study next

Pipes and cisterns with an emptying pipeWork problems where a worker leaves or joins midwayEfficiency ratios where one pipe is k times as fast as another

Common traps

Doubling A's filling time (to 32 minutes) instead of its rate. A faster pipe needs less time, 8 minutes alone.
Reporting the total time from the start, 8 minutes, when the question asks for the minutes after the change.

19 Sep 2025, 9:00, Quant Q.5 reverses the change: pipes of 18 and 24 minutes run for 6 minutes, then A halves and B doubles, and the remaining 30 of 72 units at 8 units⁄min take 3 min 45 sec.

Related PYQs

No directly related past PYQ was found.

Topic: Time & Work — all 63 SSC CGL questions on it, every solved shift.