Finding the start time, step by step

You know when something ended and how long it ran — so when did it begin?

The question

4:30 − 25 min

4:054:104:154:204:254:30 4:30
  1. It ends at 4:30 after 25 minutes. Hop BACKWARDS the same way; counting back is what stops the hour from being borrowed.
  2. It all fits inside the one hour, so it is a single hop of 25 minutes backwards — no o'clock to cross.
  3. So it started at 4:05.

How it works.

  1. 01

    It ends at 4:30 after 25 minutes. Hop BACKWARDS the same way; counting back is what stops the hour from being borrowed.

  2. 02

    It all fits inside the one hour, so it is a single hop of 25 minutes backwards — no o'clock to cross.

  3. 03

    So it started at 4:05.