The following figure shows the image of a clock in a plane mirror: Which one of the following is the correct time?

Correct — D, 9:25. A plane mirror reverses left and right, so a clock and its reflection are related by subtracting the shown time from 11:60 — that is, from 12 o'clock. The reflection printed here reads 2:35: its short hand sits between the two and the three, and its long hand points at the seven. Subtracting gives 11:60 − 2:35 = 9:25, which is the real time on the clock. You can check it the other way round: at 9:25 the hour hand lies between nine and ten on the left of the dial and the minute hand points at the five on the lower right, and reflecting that left-to-right puts the hour hand between two and three and the minute hand at the seven — exactly the figure printed.

• (a)2:35 — This is the time the mirror image itself shows, read straight off the figure without reversing it. It is the trap the question is built around — the stem says the figure IS the reflection, so 2:35 is the thing to convert, not the answer.
• (b)3:45 — 3:45 would reflect to 8:15, which would put the short hand near eight and the long hand at the three. Neither hand in the figure is anywhere near those positions.
• (c)9:15 — Close to the answer but the minute hand is wrong. 9:15 reflects to 2:45, which needs the long hand pointing at the nine, horizontally left. The printed long hand points down-left at the seven, which is 25 minutes past, not 15.

Mirror reflection of a clock face. A plane mirror reverses the horizontal axis, so a time T appears as 12:00 − T, computed as 11:60 − T to keep the arithmetic in hours and minutes.

The item cannot be answered from its stem, which only says a figure shows a clock in a plane mirror. The clock face is the data, and this question was unpublished on this site until the figure was recovered from the printed booklet.

• Mirror time = 11:60 − actual time; the rule is symmetric, so it converts in either direction.
• Write 12:00 as 11:60 before subtracting, so minutes never go negative.
• A time and its mirror image sum to 12 hours exactly — 9:25 + 2:35 = 12:00.

• Reading the reflected dial as if it were the real clock — the single most common error, and it is offered here as option (a).
• Subtracting from 12:00 without borrowing, which turns 12:00 − 2:35 into an impossible 10:-35 rather than 9:25.

Mirror-image clock items appear in the CAPF and CDS reasoning sections. They are almost always answerable in one step once you commit to the 11:60 subtraction, and almost always offer the unreversed reading as a distractor.

No directly related past PYQ was found.

• practice — not a real PYQ

  A clock seen in a plane mirror appears to show 4:20. What is the actual time?

  • (a)7:40
  • (b)8:40
  • (c)7:20
  • (d)8:20

  Answer(a) 7:40