Video: Calculating Start Times

It is 35 minutes after 9. Pick the time it was 15 minutes ago.

01:48

Video Transcript

It is 35 minutes after nine. Pick the time it was 15 minutes ago. Was it 9:20, 9:15, or 9:50?

We’re shown two different clocks. The second clock shows the end time after 15 minutes have passed. But we don’t know the start time. We know that it’s 9:35 after 15 minutes have passed. So, to find the time 15 minutes ago, we need to count back from 9:35 15 minutes. If we count back five minutes, the time will be 9:30. And we count back another five minutes; it’s 9:25. We’ve counted back 10 minutes so far. And if we count another five minutes, we’ve counted back 15 minutes altogether. And the time on the clock now says 9:20. We know that 35 take away 15 is 20. So, 35 minutes take away 15 minutes is 20 minutes.

Which of our three clocks shows the time 9:20? If it’s 35 minutes after nine, the time 15 minutes ago was 9:20. To find the answer, we counted back three lots of five minutes because three fives are 15. The time 15 minutes ago was 9:20.

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