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The Best Card Trick?

Time for a little mathemagic! Choose any five cards from a pack and show four of them to your partner. How can they work out the fifth?

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Secondary Cipher Challenge Part 1

Here is the start of a six-part challenge. Can you get to the end and crack the final message?

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Transposition Cipher

Can you work out what size grid you need to read our secret message?

Secret Transmissions

Stage: 4 Challenge Level: Challenge Level:2 Challenge Level:2

Donald, from Verulam, had a good idea to try to ensure the message went through uncorrupted:

Repeat the original message after the true message and the check digit. For example, if 1110 is the original, and 11101 is the original and check, then 111011110 is the original and check and original again. Transmit this message. We can split this up, 1110 1 1110. If each number is changed, e.g. 1111 1 1110, then we look at the 1st original message '1111' and the 2nd original message '1110'. If they are the same, then this is probably the correct message. If not, then look at the check digit.

This question was a 'Toughnut' until Michael sent in this excellent solution. Click here to see it. Well done Michael!