# Advent calendar 2018

## Archive

Show me a random puzzle**Most recent collections**

#### Sunday Afternoon Maths LXVII

Coloured weightsNot Roman numerals

#### Advent calendar 2018

#### Sunday Afternoon Maths LXVI

Cryptic crossnumber #2#### Sunday Afternoon Maths LXV

Cryptic crossnumber #1Breaking Chocolate

Square and cube endings

List of all puzzles

## Tags

dates cube numbers planes polygons arrows shape coins chocolate multiples trigonometry palindromes area bases square roots integers factors chalkdust crossnumber colouring division geometry logic crosswords triangle numbers digits indices dice quadratics dodecagons folding tube maps sport scales star numbers surds sums means remainders hexagons averages numbers angles triangles chess prime numbers rectangles shapes cryptic clues factorials perimeter square numbers balancing doubling time cards percentages coordinates irreducible numbers grids 3d shapes unit fractions taxicab geometry lines parabolas algebra ave circles volume perfect numbers clocks differentiation mean squares integration sequences number spheres ellipses games addition floors people maths routes graphs christmas partitions complex numbers odd numbers menace 2d shapes crossnumbers money wordplay probability pascal's triangle proportion advent speed probabilty symmetry cryptic crossnumbers regular shapes books functions rugby calculus sum to infinity fractions multiplication## 13 December

There is a row of 1000 lockers numbered from 1 to 1000. Locker 1 is closed and locked and the rest are open.

A queue of people each do the following (until all the lockers are closed):

- Close and lock the lowest numbered locker with an open door.
- Walk along the rest of the queue of lockers and change the state (open them if they're closed and close them if they're open) of all the lockers that are multiples of the locker they locked.

Today's number is the number of lockers that are locked at the end of the process.

Note: closed and locked are different states.