[bonanova]

Make 21, using only the four basic mathematical operators [+, -, x, /] on the numbers 1, 5, 6 and 7.

[Guest]

How many times can use each operation???

[unreality]

I take it as if we must use each of the four numbers exactly once, though we can use any of the operators as many times or not at all. Right, bonanova?

[Guest]

Make 21, using only the four basic mathematical operators [+, -, x, /] on the numbers 1, 5, 6 and 7.

Been working on it for the last hour solid ( at work )

6 / (1 - 5/7)

[Prime]

Make 21, using only the four basic mathematical operators [+, -, x, /] on the numbers 1, 5, 6 and 7.

How about 1+5+6+7+1+1=21? Or did you mean use each number exactly once and each operator as you pleased?

[unreality]

in that case, 7+7+7 ;D or 7*(1+1+1) or 7*(6-5+1*(7-5)) or anything else ;D See my post #3

[Prime]

in that case, 7+7+7 ;D or 7*(1+1+1) or 7*(6-5+1*(7-5)) or anything else ;D See my post #3

You are not allowed to use "()" -- only "[]". See the statement of the problem.

[unreality]

You are not allowed to use "()" -- only "[]". See the statement of the problem.

the brackets were to enclose what you could use. I assumed you can use as many parenteses as required for the order of operations

[Prime]

Here is one possible way:

5+6+7-1=21 in base 8 system, of course.

[bonanova]

Been working on it for the last hour solid ( at work )

6 / (1 - 5/7)

taliesin has it.

Nice job.

[Guest]

taliesin has it.

Nice job.

Thanks

[Prime]

taliesin has it.

Nice job.

Agree on that.

Still, I think, my solution (post 9) is valid too.

[Guest]

Here is one possible way:

5+6+7-1=21 in base 8 system, of course.

I think you deserve 10 points for that!

[bonanova]

I think you deserve 10 points for that!

But 10 octal is just an 8 ...

[Prime]

But 10 octal is just an 8 ...

Do you mean, you asked to find decimal 21, not octal!? What are the chances of that?

[Guest]

Ok first off I've never even heard of base 8 system lol.

And I didn't know you could use parentheses.

If you had said that it would have made it a whole lot easier Bonanova.

[Guest]

15 + 6

[Prime]

15 + 6

I thought of that. But the tribunal decided that you have to use each number exactly once. And you didn't use "7".

[bonanova]

Ok first off I've never even heard of base 8 system lol.

And I didn't know you could use parentheses.

If you had said that it would have made it a whole lot easier Bonanova.

The following statement may or may not be false.

The previous statement definitely is false.

After you figure that out, note that the OP limits the mathematical operators that you can use to "+,-,*,/."

Contrarily, parens are just signposts. Yes, they can affect the calculation, but intrinsically, they're not ops.

For example 5+6 = 11 = 5+(6) = (5)+6 = (5) + (6) = (5 + 6) = ((5) + 6) = ...

[Guest]

The following statement may or may not be false.

The previous statement definitely is false.

After you figure that out, note that the OP limits the mathematical operators that you can use to "+,-,*,/."

Contrarily, parens are just signposts. Yes, they can affect the calculation, but intrinsically, they're not ops.

For example 5+6 = 11 = 5+(6) = (5)+6 = (5) + (6) = (5 + 6) = ((5) + 6) = ...

Also, if you think about it, when you write it on paper, the solution to this problem definitely doesn't require any parentheses. They are just there to help when it is all laid out on one line of a forum post.