Maths

Can you count down from twenty to zero?

Can you count down from a hundred in tens?

Can you do sums like eighteen minus nine? (Test yourself by clicking here now.)

If not, then you need to find someone who can teach you how to do these things. Otherwise read on:-

Here we go then! Big take-away sums are written out in the same way as adding sums, but the bottom number is to be taken away from the top one. We'll start with 43 - 31:-

  43
- 31
    

Click on these two buttons to solve the sum: 3 - 1 = 2 (the column on the right is done first), 4 - 3 = 1 (and then the other column is done). When doing the second column we are actually doing the sum 40 - 30 = 10, even though it looks like 4 - 3 = 1. Once both columns have been done, the number at the bottom between the two lines, 12, is the answer to the whole sum, so 43 - 31 = 12. The method is very similar to adding: you do the right-hand column first, and then the column to the left of it. Let's do another one, but with much bigger numbers (3597 - 1024):-

  3597
- 1024
      

This time there are four columns to do: 7 - 4 = 3; 9 - 2 = 7; 5 - 0 = 5; and 3 - 1 = 2. If you have clicked all four buttons, you can now see the answer: 3597 - 1024 = 2573.

Let me remind you: if you're reading this on a computer with a small screen you'll be able to see much more of the page at a time by pressing the F11 key. Pressing that same key again will return things to normal afterwards.

So taking away big numbers is as easy as adding, but again there is a little problem, though this time it's a lot easier to spot:-

  3629
- 2351
      
  

Click these four buttons: 9 - 1 = 8; 2 - 5 = ?; 6 - 3 = 3; 3 - 2 = 1. But what use is this? We don't have a proper answer because we can't take 5 away from 2. The real answer for the sum is actually 1278; not 13?8. To solve the sum properly, click these buttons: ; 9 - 1 = 8; 12 - 5 = 7; 6 - 3 - 1 = 2; 3 - 2 = 1.
indentThe orange 1 is now used differently from the way it would be if we were adding. It is to be taken away from the column it has been written in, and the reason for taking it away from that column is that it gives us an extra 10 to play with in the problem column: we can add it to the 2 to turn it into 12, making it big enough to take the 5 away. Notice that because we are taking away and not adding, the orange 1 combines with the 2 at the top to make 12: it does not combine with the 7 to make 17. Click the reset button and go through it all again, making sure you understand exactly what happens as you press each button.

Now that you have seen how to deal with the problem of taking a bigger number from a smaller one, you can solve any take-away sum. Here's a big one for you to try yourself. See if you can work out the answer for each column before you press the button for it. If you click the wrong button, just click the reset button and start again:-

  4027
- 2444
      
  

7 - 4 = 3; 12 - 4 = 8; 10 - 4 - 1 = 5; 4 - 2 - 1 = 1. To start again, click: . Click through the sum several times to make sure you understand it properly. Don't be alarmed by the purple 1: it only changes to that colour when you press the third button to help you understand which 1 is taken away and which 1 combines with the 0 to make a ten.

Now that you have seen how taking away should be done, you need to fix the method in your head by doing lots of sums: click here for a game which will help you do that. At some point, you will also need to try doing take-away sums on paper instead of clicking buttons on a computer to show that you really can do it properly.