Math this week

 

We will continue to work on two-digit subtraction. This week, we will be introducing subtraction with regrouping.

A conventional algorithm for subtracting is not the only way to subtract multidigit numbers, but it is a traditional method that many adults are familiar with.

This traditional procedure for subtracting whole numbers involves takeaway thinking. For two-digit subtraction, first the ones of the smaller number are taken from the ones of the greater number, and then the tens of the smaller number are taken from the tens of the greater one.

These steps can be modelled with base ten blocks. The steps in a conventional algorithm mirror the actions completed using the blocks.
For example, to subtract 53 – 21:
Subtract 1 one from 3 ones and 2 tens from 5 tens.

Sometimes, though, there are not enough ones in the greater number to take from, so you regroup: trade 1 ten from the greater number for 10 ones.

For example, when you subtract 52 – 38, there are not enough ones in 52 to remove 8 of them.

Step 1:
Trade 1 ten for 10 ones. Rewrite 52 as 40 + 12 (4 tens and 12 ones).

Step 2:
Subtract 8 ones from 12 ones. There are 4 ones remaining.

Step 3:
Subtract 3 tens from the 4 tens that are left, leaving only 1 ten. The answer is 14.

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Watch for times when you might want to subtract two-digit numbers, for example, when comparing scores at the end of a game or when estimating your savings on a discounted item. You might share how you do the subtraction so that your child can either watch you do it or help you do it.
Always remember, though, that any subtraction strategy that works is absolutely fine. What matters is whether the procedure makes sense to your child.

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We no longer use the term “borrow.” One of the reasons is that too many students wanted to “give back” what they borrowed. This led to getting the wrong answer.
Instead we talk about regrouping 1 ten into 10 ones.
It is not necessary to use the term “algorithm” with your child; you could refer to the “standard strategy” if you want to name it.