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# Subtraction by decomposition!

February 12, 2013

#### By Will

This is the way we do subtraction in our school. We use special language such as renaming, hundreds, tens and units. Please ask any of our mathematicians to explain it to you! It may look tricky, but it makes sense! Heres what we do in our class: • We always start with the units: 2 minus 8 we cannot do!!, so we need to go over to the tens, take one of them and rename it into units! Since we took one of the tens, we have to cross it out and say how much is left: we had 4 tens, so now we have 3 left and we write this number beside the old tens. Now we can bring over our ten, and rename it into units – (see green arrow). Now we have 12 units.
• We are now ready to subtract the normal way: 12units minus 8 units = 4units. We put this 4 under the units column.
• Now we are ready to subtract our tens: 3tens minus 2tens = 1 ten.
• Now we have our answer, 14!

Now heres a trickier sum, with hundreds tens and units. But if we stick to the same process, it will be no problem to us! Have a look at what happens if we have to rename when there are no tens:

• Start with the units: 3 minus 8, we cannot do!! (shout this!) so we go to the tens but wait!!
• There are no tens!
• We must now go to the hundreds and rename one into tens (see blue arrow)
• Now we do have tens. So we start again with the units
• 3 minus 8, WE CANNOT DO. So lets go to our tens: this time there are tens there (10 tens!). We take one, and there are 9 left. We can now rename that ten into units and we end up with 13 units minus 8 units = 5
• Next we subtract our tens: 9 – 7 = 2
• and finally our hundreds: 2 – 1 = 1 