Algorithms

When the number x 1 x x x is multiplied by 417 this gives the answer 9 x x x 0 5 7. Find the missing digits, each of which is represented by an "x" .

Imagine a strip with a mark somewhere along it. Fold it in the middle so that the bottom reaches back to the top. Stetch it out to match the original length. Now where's the mark?

How would you judge a competition to draw a freehand square?

Third challenge cipher

Fourth challenge cipher

Fifth challenge cipher

Seventh challenge cipher

Can you crack these very difficult challenge ciphers? How might you systematise the cracking of unknown ciphers?

Second challenge cipher