SSC CGL 2024 Tier-I · 11 September 2024, Shift 3 · Quantitative Aptitude · Question 10 of 100

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.

A boat can go 54 km upstream in 6 hours. If the speed of the stream is 4.8 km/h, then how much time (in hours) will the boat take to cover a distance of 279 km downstream?

(a)18
(b)15
(c)16
(d)20

Answer

Why

Correct — B. The first sentence fixes the upstream speed, and everything else follows from it.

Upstream speed = 54 ÷ 6 = 9 km/h
Still water = upstream + stream = 9 + 4.8 = 13.8 km/h
Downstream = 13.8 + 4.8 = 18.6 km/h

Time = 279 ÷ 18.6 = 15 hours → option (b)

The shortcut is the same arithmetic in one line: downstream = upstream + 2 × stream = 9 + 9.6 = 18.6.

Why the others are wrong

(a)
18 — 18 hours would need a downstream speed of 279 ÷ 18 = 15.5 km/h. The only speeds this stem builds are 13.8 in still water and 18.6 downstream, and 15.5 is neither.
(c)
16 — 16 hours would need 279 ÷ 16 ≈ 17.44 km/h. That is neither the still-water 13.8 nor the downstream 18.6 but a value between them, so it cannot come from adding the stream the right number of times.
(d)
20 — This is the still-water trap. 279 ÷ 13.8 ≈ 20.2, so 20 is what you get by rowing downstream at the boat's own speed and forgetting that the current adds a further 4.8 km/h.

Concept

Call the boat's still-water speed b and the stream s. Then upstream = b − s and downstream = b + s.

A stem that gives you an upstream figure has already subtracted the stream once. To reach downstream you must add it back and then add it again — the stream is worth 2s across the two directions.

So downstream = upstream + 2s = 9 + 9.6 = 18.6 km/h, and the only thing left is one division.

The numbers are chosen to divide cleanly: 54 ÷ 6 = 9 and 279 ÷ 18.6 = 15. A decimal that will not cancel usually means the still-water step has gone wrong.

Key facts

Upstream speed is the still-water speed minus the stream speed.
Downstream speed is the still-water speed plus the stream speed.
Downstream speed = upstream speed + 2 × stream speed.
54 km in 6 hours upstream fixes the upstream speed at 9 km/h, so still water is 13.8 km/h.

Study next

Boat-and-stream problems that give the two times and ask for the two speedsWhy downstream time to upstream time over one distance equals (b − s) to (b + s)Relative speed on land: two bodies moving in the same and opposite directions

Common traps

Adding the stream once to the upstream speed and calling that the downstream speed.
Treating the 9 km/h upstream figure as the boat's speed in still water.
Rounding 18.6 to 19 before dividing, which breaks the clean 15.

This stem hides the upstream speed inside a distance and a time, and hands the stream speed over outright. 10 Sep 2024, 16:00, Quant Q.12 hands over still water 18 km/h and stream 3 km/h and asks only for a time.

17 Sep 2024, 12:30, Quant Q.12 runs in reverse — 18 hours downstream and 36 hours back, find the ratio of boat speed to stream speed. 23 Sep 2024, 16:00, Quant Q.9 and 13 Sep 2024, 09:00, Quant Q.22 give both journeys and ask you to separate b from s.

Related PYQs

No directly related past PYQ was found.