Brain Teasers
Five 5`s
How can five 5`s equal 31?
Answer
5x5+5+5/5 (or five times five plus five plus five-fifths)=31Hide Answer Show Answer
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Comments
Nov 21, 2001
Or use base 8
base 10
5+5+5+5+5 = 25
base 8
5+5+5+5+5 = 31
base 10
5+5+5+5+5 = 25
base 8
5+5+5+5+5 = 31
How does base 8 work?
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. twenty-four. 31 is twenty-five. 100 is eight times eight, i.e. sixty-four.
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.
5 X 5 + 5 + .5 + .5 = 31
Oh well, it was fun anyway.
Subtox, that was a good alternate solution!
(Old math teacher!)
(Old math teacher!)
I got the expected solution, but I always like to get one extra:
(5! + 5 + 5) / 5 + 5 = 31
(5! + 5 + 5) / 5 + 5 = 31
5^2+5^1+5^0
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