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More Secret Transmissions
Age 16 to 18
Challenge Level 
In Secret Transmissions, the check digits were in position 1, 2 and 4 of the 7 digits, and the message digits were in position 3, 5, 6 and 7.
Digits 1, 3, 5 and 7 contained an even number of 1s.
Digits 2, 3, 6 and 7 contained an even number of 1s.
Digits 4, 5, 6 and 7 contained an even number of 1s.
What do you notice about the position of the check digits in the message?
Where might you put the next check digit in a longer message?
How can you determine which message digits 'belong' to each check digit?
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