Count back
How well can you count back from sixty thousand?
Age
11 to 14
Challenge level
Problem
What number is twenty-one less than sixty thousand?
Student Solutions
Answer: 59979
Subtracting 1 first
Sixty thousand = 60 000.
60 000 - 1 = 59 999.
59 999 - 20 = 59 979.
Partitioning sixty thousand
Sixty thousand = 60 000.
60 000 = 59 000 + 1000
= 59 000 + 900 + 100
= 59 000 + 900 + 99 + 1
= 59 + 900 + 79 + 20 + 1
So 60 000 - 21 = 59 000 + 900 + 79 + 20 + 1 - 21
= 59 000 + 900 + 79
=59979
Column method
The sum can be written in columns, as shown below.
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$0-1$ is negative. Borrow from the $6$, because nothing can be taken from any of the zeros.
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Now $1$ can be moved from the '$10$' in the second column to leave $9$:
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Continuing in this way gives
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And then subtracting,
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