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← The Pool Deck Is a Maths Room · Lesson 2 of 4

Hundredths, and a borrow of sixty

About it

Swim times are written minutes, colon, seconds, point, hundredths: 1:05.32 is one minute, five seconds and thirty-two hundredths. The colon and the point look alike on a scoreboard, and they are not the same kind of number.

Taking one time from another trips people up. 1:05.32 take away 58.90 is not 46.42. Borrow a minute, which is sixty seconds, not a hundred: 65.32 − 58.90 = 6.42 seconds.

The figure

Borrow a minute, which is sixty seconds

1:05.32 is 65.32 secondsone minute = 60 seconds5.3258.906.42 sread the colon as a point and you get 46.42, which is wrong
Two bars on one scale of seconds. 1:05.32 is one minute, sixty seconds, plus 5.32: 65.32 seconds in all. Below it, 58.90 seconds. The part of the top bar left over is 6.42 seconds, the difference.
One question

Last month he swam 1:02.15. Today he swam 57.80. How much faster is he?

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