A clock strikes once at 1 o’clock, twice at 2 o’clock and thrice at 3 o’clock, and so on. If it takes 12 seconds to strike at 5 o’clock, what is the time taken by it to strike at 10 o’clock?
Answer & explanation
Answer: (b) 24 seconds
Why not the tempting option · UPSC's key, 24 seconds, treats the time as proportional to the number of strikes (2.4 s a strike). The textbook method counts the gaps between strikes — 4 gaps in 12 s, so 9 gaps at 10 o'clock make 27 s — but 27 is not among the options, so the paper plainly intends the simple proportion. In the exam, if the gap method gives no option, switch to proportion; (b) is the only option that fits the data.
UPSC's key takes the striking time as proportional to the number of strikes: 5 strikes take 12 seconds, so each strike accounts for 2.4 seconds and 10 strikes take 24 seconds. This is the only option that fits the data.
- Read the data as 'each strike takes the same time': 12 s ÷ 5 strikes = 2.4 s a strike.
- At 10 o'clock the clock strikes 10 times: 10 × 2.4 = 24 s, option (b).
- Check the alternative: counting only the gaps between strikes (4 gaps of 3 s, then 9 gaps) gives 27 s, which is not offered, so that reading cannot be what the paper intends.
Remember · In a clock-strike question, first try the gap method (strikes − 1); if the result is not among the options, use simple proportion, as UPSC did here.
Question and answer: UPSC's official GS Paper II (2017, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 1 Oct 2026 (how we verify). ·