Hours, minutes and the calendar · Usually met around P2
Timekeepers old and new
An hourglass always takes the same time.
How to do it
Method
An hourglass always takes the same time
A 3-minute glass 4 times: 3 × 4 = 12 minutes
Another way
A time line
0 → 3 → 5: 5 − 3 = 2
In full
Long ago, people told the time from a sundial's shadow and measured stretches of time with hourglasses and water clocks; later came pendulum clocks, watches and digital clocks. An hourglass takes the same time each time it runs out: a 3-minute one turned over and over, run out 4 times, is 3 + 3 + 3 + 3 = 12 minutes. Two together can measure other times: start a 3-minute and a 5-minute glass together; from when the 3-minute one runs out to when the 5-minute one does is 5 − 3 = 2 minutes. Or draw a time line and mark when each runs out.
Worked examples
Have a go first, then open each one for the answer and the working.
Example 1
To time 20 minutes with a 5-minute hourglass, how many times must it run out?
Show the answer and working
Answer: 4 times
5 minutes a time: counting in 5s to 20 takes 4.
Example 2
A 5-minute hourglass running out 6 times makes 11 minutes.
True or false?
Show the answer and working
Answer: False
6 lots of 5 minutes is 30.
Example 3
A 3-minute and a 7-minute hourglass are started together. From when the 3-minute one runs out to when the 7-minute one does, how many minutes pass?
Show the answer and working
Answer: 4 minutes
One runs out at minute 3, the other at minute 7: 7 − 3 = 4.
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