SSC CGL 2024 Tier-I · 24 September 2024, Shift 2 · Quantitative Aptitude · Question 5 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 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.

Two goods trains 132 m and 108 m in length are running towards each other on parallel tracks. The first train is running at a speed of 32 km/h and the second at a speed of 40 km/h. How much time will they take to cross each other after meeting?

(a)12 s
(b)15 s
(c)10 s
(d)13 s

Answer

Why

Correct — A. Running towards each other, the two speeds add.

Total length to be cleared = 132 + 108 = 240 m
Relative speed = 32 + 40 = 72 km/h
Convert: 72 × 5⁄18 = 20 m/s
Time = 240 ⁄ 20 = 12 s → option (a).

Why the others are wrong

(b)
15 s — 15 s is 240 ⁄ 16, so it assumes the trains close at 16 m/s. Adding the given speeds gives 72 km/h, which is 20 m/s.
(c)
10 s — 10 s is 240 ⁄ 24 and would need a closing speed of 86.4 km/h. The trains are doing 32 and 40, which sum to 72.
(d)
13 s — 13 s needs about 18.5 m/s. The exact figure is 20 m/s, and 240 ⁄ 20 divides cleanly, so there is nothing here to round.

Concept

Two trains crossing each other must clear the sum of their lengths. The fronts meet first, and they are only clear of each other once the rear of one has passed the rear of the other — 132 + 108 metres of travelling in all.

Direction decides how the speeds combine. Towards each other, add; same direction, subtract.

The conversion is what most of these items really test: 1 km/h = 5⁄18 m/s, so 72 km/h is exactly 20 m/s.

The phrase "after meeting" is not an extra condition. It simply marks the instant the fronts are level, which is where the 240 m starts being counted.

Key facts

Time to cross = (sum of the two lengths) ÷ (relative speed).
Relative speed is the sum for opposite directions and the difference for the same direction.
Multiply km/h by 5⁄18 to get m/s, so 72 km/h is 20 m/s.

Study next

A single train crossing a pole, a platform or a bridgeTwo trains in the same direction, where the relative speed is the differenceBoats and streams, which use the same add-or-subtract logic upstream and down

Common traps

Using one train's length instead of the sum of both
Subtracting the speeds, which is the same-direction case
Dividing metres by a speed still written in km/h

SSC gives both lengths and both speeds and asks for the time, or reverses it. At 19 Sep 2024, 09:00, Quant Q.21 the two crossing times are supplied — 20 s in the same direction and 5 s in opposite directions — and the speeds are asked for.

Either way the decision that carries the mark is whether the speeds add or subtract.

Related PYQs

No directly related past PYQ was found.