# Tens Complement

**What Makes 10?**

The "Tens Complement" is the number we add to make 10:

Example: The Tens Complement of **3** is **7**, because **3+7=10**

Here are all the Tens Complements:

Number | Tens Complement | Why? Because: |
---|---|---|

0 | 10 | 0 + 10 = 10 |

1 | 9 | 1 + 9 = 10 |

2 | 8 | 2 + 8 = 10 |

3 | 7 | 3 + 7 = 10 |

4 | 6 | 4 + 6 = 10 |

5 | 5 | 5 + 5 = 10 |

6 | 4 | 6 + 4 = 10 |

7 | 3 | 7 + 3 = 10 |

8 | 2 | 8 + 2 = 10 |

9 | 1 | 9 + 1 = 10 |

10 | 0 | 10 + 0 = 10 |

But you only have to remember half the table!

**1** + **9** = 10 and **9** + **1** = 10

**1 and 9** are partners!

So are **2 and 8**, **3 and 7**, **4 and 6** and **5 and 5**

## Why is this Important?

Knowing the Tens Complements makes some mental arithmetic easier.

### Example: 25 + 8

Start with:25 + 8

8 and 2 make 10, so:25 +

**10****− 2**
Add the 10:35 − 2

Subtract the 2:33

In practice we think quickly like this:

"8 is 2 away from 10": **25+10 is 35, **−**2 is 33**

### Example: 74 − 9

"9 is 1 away from 10": **74****−10 is 64, +1**** is 65**

It is also used in Quick Subtraction.

## Practice!

You can practice by using What Makes 10?