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.

(In reply to re: one solution by Sing4TheDay)

correct you are, so I decided to write the program to find all solutions and the only ones I found were

6/(1-5/7)=21

and

-(6/(5/7-1))=21

second only counts if we are able to use the minus sign in its unary form of negations.  Of course allowing that seems to only give us a trivial variation on the first solution.

Posted by Daniel on 2009-03-12 12:46:02