How do you make 21 using using 1, 5, 6 and 7?

How do you make the number 21 using 1, 5, 6 and 7 and standard arithmetic operations (addition, subtraction, multiplication and division). You can also use parentheses.

You CANNOT combine two numbers. For example, 15 + 6 = 21...you can't combine 1 and 5 to make 15.

There are no complex solutions to this, but for some reason many people have a lot of trouble with this problem. Don't tell me it's impossible because I have the answer.

Also, if you already know the answer, let other people figure it out on their own.

Update:

all of these are wrong so far...and no you cannot use a number

Update 2:

you cant do stupid tricks with numbers like turn them around...this is all simple arithmetic!!!

Update 3:

USE EACH NUMBER ONCE!

Update 4:

I have given you all the information needed...If I tell you the answer, you'll be hitting yourself on the head when you realize that it was not that hard.

Update 5:

AHH! I hate people who think that every problem can be solved by just looking at it...NO, it's not that easy, or else I wouldn't have asked it..Those of you who are saying it's impossible, check back in a couple of days to see the answer

Update 6:

I'm not changing the rules, I just forgot a few the first time I wrote the question...do you think I'm like trying to trick you or somethin??

Update 7:

ADDITION, SUBTRACTION, MULTIPLICATION, DIVISION and PARENTHESES...read the question

Update 8:

no, sqrt usually isn't considered part of addition, subtraction, multiplication, division and parentheses

7 Answers

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  • 1 decade ago
    Best Answer

    But this one is CORRECT:

    6/(1-(5/7))

    WOO HOO!!!!!

    Source(s): 2+ hours and a master's degree in math is paying off! :-)
  • 3 years ago

    6/(1-5/7)

  • 1 decade ago

    ((5-1) x 7) - 6 (this equals 22 . . . close!)

    oops . . still working on it . . .

    Using base 9: 5+6+7=20 . . .then +1 = 21 !!!!!!

    OR . . how about: 5 x 6 x 7 x .1=21 !!!!!!!!!

    You can't keep changing the rules after someone answers it !

    I now discover this has been presented in this forum before several times . . . and there has been no correct answer provided by the person asking the question . . . I have presented two clear and correct answers . . unless you want to change the rules again this has been solved here.

  • 1 decade ago

    uhmmm. there is a loop hole in your directions, u didnt tell us how many times we could use each number so we could do:

    5+5+5+5+1=21 or

    5+5+5+7-1=21 or

    7+7+7=21 or

    6+6+6+1+1+1=21 or an endless number of combinations

    if this is not the solution, then i have no clue....

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  • Anonymous
    1 decade ago

    7+5=12 turn it around=21.

  • 1 decade ago

    You're not telling us something..... or else many people would've gotten it already.

    So therefore, I will be the first to say IT'S IMPOSSIBLE.

  • 1 decade ago

    this is ridiculous

    i doubt that there really is an answer

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