1 | 55 | 28 | |||

- | 12 | 10 | |||

1 | 43 | 18 |

It's easier to think “28-10=18” and “55-12=43”

Carries still apply as normal:

4812 - 1598

48 | 12 | |||

- | 15 | 98 | ||

32 | 14 | A bit like school, 12 – 98, make it 112 – 98 = 14 carry one | ||

1 |

Do 48-15 then subtract one, 48-15=33, 33 – 1 = 32

52 | |

- | 19 |

I think, if I slide the 19 up one to 20, then slide the 52 up one to 53, it becomes:

53 | |

- | 20 |

33 |

Another example 84 - 68:

84 | |

- | 68 |

I think, if I slide the 84 up two to 86, then slide the 68 up two to 70, it becomes:

86 | |

- | 70 |

16 |

If you know someone was born in 1979 and you want to find out how old they are, you have:

2008 – 1979

Well, a nice number between these is the number 2000.

First of all, work out the differences between 2000 and 2008, and 1979 and 2000, and then add the differences up

The difference between 2000 and 1979 is 21

The difference between 2000 and 2008 is 8

Add the differences: 21 + 8 = 29

12 | • | 58 | |

- | 6 | • | 20 |

6 | • | 38 |

1100 - 150 → First of all, subtract the 100 to make it easier:

1100 - 150 = 1000

now all you have to do is subtract the remaining 50:

1000 - 50 = 950

Another example:

1450 - 125

I can either subtract the 100, the 20 and the 5 separately, but looking at it, I have decided to subtract the 100 and the 25 instead:

1450 - 100 = 1350

13 50 - 25 = 13 25 (I split them up in to pairs so I can think about them more easily, as explained above.

Just to explain last bit, I thought in my head:

13 | 50 | |

- | 25 | |

25 |

This method is like the 'sliding rule' above, only thought about in a different way.

-4 | -3 | -2 | -1 | 0 | 1 | 2 | 3 | 4 | 5 |

So for the number 2 – 5, you would start at 2, and then count down 5 until you get to -3. To add, count to the right -3 + 4 → -3, -2, -1, 0, 1

So -3 + 4 = 1.

Another way to think of them is a positive number is like being in credit and negative is like being in debt.

Let's look at 2 – 5 again. If you had £2 in a bank and you withdrew £5, you will be £3 in debt or 2 – 5 = -3. If you have £3 of debt and you put in a cheque for £4 then you will have £1 in the bank so -3 + 4 = 1.

Have you found an error or do you want to add more
information to these pages? You can contact me at the bottom of the home page. |