Five 5`s
Math brain teasers require computations to solve.
How can five 5`s equal 31?
Answer
5x5+5+5/5 (or five times five plus five plus fivefifths)=31
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(user deleted)
Nov 21, 2001
 Or use base 8
base 10
5+5+5+5+5 = 25
base 8
5+5+5+5+5 = 31

thebuddhawithin
Jul 04, 2004
 How does base 8 work? 
canu
Jul 14, 2004
 In base 8, each place to the left is worth 8 times more than the one to its right (in the usual notation it's 10 instead of .  So, in base eight, the only digits are 0 1 2 3 4 5 6 7. Eight is written 10. 30 is three times eight, i.e. twentyfour. 31 is twentyfive. 100 is eight times eight, i.e. sixtyfour.

Subtox
Mar 18, 2007
 I got 31 by placing a decimal point in front of two of the fives, but I'm pretty sure that wasn't allowed...
5 X 5 + 5 + .5 + .5 = 31
Oh well, it was fun anyway.

mosca
Jun 26, 2007
 Subtox, that was a good alternate solution!
(Old math teacher!) 
javaguru
Jan 28, 2009
 I got the expected solution, but I always like to get one extra:
(5! + 5 + 5) / 5 + 5 = 31

xdbtcp
Dec 15, 2014
 5^2+5^1+5^0

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