Alonzo Church 

Alonzo Church, the creator of lambda calculus, is only 17 in 1920. He is making himself a fake ID. He needs it to say he is 21. The problem is he cannot use a computer. Rather, he has some symbols he bought from the store, and that’s it.

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He has one 1, one 5, one 6, one 7, and an unlimited supply of +, –, *, /, (, and )  

(the operations addition, subtraction, multiplication and division)

 

Using each number exactly once (but you can use any number of +, any number of -, …) how can he get 21 from 1, 5, 6, 7?

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