My daughter in grade 3 is learning about telling time at her school. She eagerly showed me this method she has discovered on her own to tell the minutes part of the time on an analogue clock. I wasn’t sure at first because I have never heard about it before but it works really well.
Here is her method:
Look at the minute hand and see what number it is pointing to, let’s say it’s 3.
Add a zero at the end to make it 30.
Halve it to get the minutes, so 3 becomes 30 and halving it gives 15, 6 becomes 60 and halving it gives 30 and so on.
It works well but I am not sure why. What mathematically justifies the method used?
Each number on the clock face is worth five minutes. One good way to multiply by 5 is to first multiply by 10, and then divide by 2. This works because 5=10÷2.