# Maths

So we've completely cracked column addition down to solving decimal problems. Let's meet it's evil twin, column subtraction! Not to panic, we've done these in class before - have a watch of this BBC Bitesize clip of a cinematic catastrophe and it'll all come back to you, like riding a maths bike with numbers instead of wheels;

https://www.bbc.co.uk/bitesize/topics/zy2mn39/articles/zc78srd

They call exchanging between place values 'borrowing' there - but it's the same thing. If the number on the top is smaller than the number on the bottom, you have to exchange, don't be tempted to do this mistake!

People see that and see the first one, think 4 - 5, that's easy! 1! But it's the wrong way round, you need to do 4 - 5. Since 5 is usually bigger than 4, that means we need to exchange.

So we'll exchange 1 from that 3 in the tens column. 3 take away 1 is normally 2, so we'll change that 3 into a 2. The 1 we 'borrowed' goes in front of the 4 to make it 14, now we have 14 - 5, which we can safely say is 9. Ok, move on to the next column, 2 - 4. Hmm. 2 is smaller than 4, so we'll have to...yep, exchange! Let's rob from next door, the 1 goes in front of that 2, now we have 12 - 4, which experts assure me is 8.

Last, we have 1 - 1, the most advanced supercomputers known to mankind will tell you that this is 0. You can put 0 in front if you want, but I've saved the ink because it doesn't change the number at all.

Put your exchanging knowledge to the test with these column subtractions:

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