# 8 Balls Weight Puzzle (Solved)

This is my favorite weight puzzle which have been asked from me in many interviews over the past few years.

## Puzzle

You have 8 balls identical in size and appearance. One of them is defective and weighs heavy than the others. You have a weighing scale with no measurements so you can just compare weight of balls against each other. How would you find the defective ball in 2 weightings?

## Solution

First of all we will give a number to each ball i.e. 1, 2, 3, 4, 5, 6, 7, and 8

The trick to solve these kind of weight problem is to divide them in groups. We will divide these 8 balls in 3 groups:

Group | Ball Numbers |
---|---|

group 1 | 1, 2, 3 |

group 2 | 4, 5, 6 |

group 3 | 7, 8 |

Now we keep the **group 3** aside and put **group 1** balls on one side of scale and **group 2** balls on another side of scale ** ^{[1]}** with three possible outcomes:-

#### 1. Scale is balanced

That means each ball in **group 1** and **group 2** are identical in weight and defective one is from **group 3**.

Let’s put **group 3** balls *7 and 8* on each side of scale** ^{[2]}** with two possible outcomes:-

- If
*7*is heavy then it is defective one - If
*8*is heavy then it is defective one

#### 2. *group 1* is heavier then *group 2*

That means defective balls is from *group 1* i.e. either *1 or 2 or 3*.

Let’s keep number *3* aside and put balls *1 and 2* on each side of scale** ^{[2]}** with three possible outcomes:-

- If
*1 and 2*balances, then*3*is defective one - If
*1*is heavy then it is defective one - If
*2*is heavy then it is defective one

#### 3. *group 2* is heavier then *group 1*

That means defective balls is from *group 2* i.e. either *4 or 5 or 6*.

Let’s keep number *6* aside and put balls *4 and 5* on each side of scale** ^{[2]}** with three possible outcomes:-

- If
*4 and 5*balances, then*6*is defective one - If
*4*is heavy then it is defective one - If
*5*is heavy then it is defective one

Reference:-** ^{[1]}** is weighting for the first time

**is weighting for the second time**

^{[2]}## Conclusion

That’s it guys. We have found the defective balls out of 8 balls in only 2 weightings.

If by now you understood the trick of dividing balls in group and keeping some balls aside then you can solve weight puzzle with any number of balls. Here is the cheat sheet:-

###### Cheat Sheet

N Balls Weight Puzzle | Groups | Min Weightings (Best Case) | Min Weightings (Worst Case) |
---|---|---|---|

N = 2 | [1] [2] | 1 | 1 |

N = 3 | [1] [2] [3] | 1 | 1 |

N = 4 | [1] [2] [3,4] | 1 | 2 |

N = 5 | [1,2] [3,4] [5] | 1 | 2 |

N = 6 | [1,2] [3,4] [5,6] | 2 | 2 |

N = 6 | [1,2,3] [4,5,6] [7] | 1 | 2 |

N = 8 | [1,2,3] [4,5,6] [7,8] | 2 | 2 |

N = 9 | [1,2,3] [4,5,6] [7,8,9] | 2 | 2 |

N = 10 | [1,2,3,4] [5,6,7,8] [9,10] | 2 | 3 |

N = 11 | [1,2,3,4] [5,6,7,8] [9,10,11] | 2 | 3 |

N = 12 | [1,2,3,4] [5,6,7,8] [9,10,11,12] | 3 | 3 |

If you are interested in how to solve 12 balls weight puzzle with a twist that you don’t know whether it is light or heavy then check out this post - How to solve 12 balls weight puzzle