Use the numbers 1, 5, 6 and 7 exactly once each, combined with as many additions, subtractions, multiplications and divisions as necessary to result in 21.

There are no tricks. Base 10 is used; digits are not combined into integers of more than one digit; no factorial or exponentiation is used. There are no square roots or modular arithmetic.

Parentheses are allowed so as to let you specify the order of the allowed operations that you select.

(6+1)*(7-5+1)=21

I'll leave it to someone else to write the computer program to determine all other solutions if there are any

Posted by Daniel on 2009-03-12 11:17:00