This may be too easy, but:Using ONLY the numbers 1, 5, 6 and 7 (and using each one ONLY ONCE), make a formula that equals 21.You can do any operation you like, but only use those numbers. You CAN NOT stick the numbers together, e.g. sticking 1 and 5 together to get 15 is NOT allowed.

Not the answer I was looking for, but it works!There is another solution.

QuoteOriginally posted by: ArrowayThis may be too easy, but:Using ONLY the numbers 1, 5, 6 and 7 (and using each one ONLY ONCE), make a formula that equals 21.You can do any operation you like, but only use those numbers. You CAN NOT stick the numbers together, e.g. sticking 1 and 5 together to get 15 is NOT allowed.You said any operation so here is one suggestion21=6*7/(Int(ln(5))+1)Int is the integer part and ln is the natural logarithm.Regards,Niclas

I don't know if "Int" is an operation, but I'm lenient, so sure that counts. There is still another way...

QuoteOriginally posted by: ArrowayI don't know if "Int" is an operation, but I'm lenient, so sure that counts. There is still another way...If one can use the Int operation and therefore relaxing the rules by a whole lotthen one can also do ceiling(6/5) = ceiling(1.2) = 2so ( ceiling(6/5)+1 )*7 = 21 too. Here ceiling is an operator that rounds a decimal number upward.

Sure you get credit too!But there is a MUCH cleaner (and more exact) way...

Last edited by Arroway on March 17th, 2004, 11:00 pm, edited 1 time in total.

QuoteOriginally posted by: ArrowaySure you get credit too!But there is a MUCH cleaner (and more exact) way...Okay what operators are there in your answer? Surely they are not restricted to { +,-, * , / , ^}

Yup, you can do it with the set of operators that includes add, subtract, multiply and divide.

Arroway,I got the "clean" solution using only four basic arithmetic operations:6 / (1 - 5/7) = 21

While I am lenient, I don't know if that one qualifies. The reason is that square root is equivalent to raising to the one half power. Obviously you need a one and a two to do that. Maybe squares and square roots are operators in and of themselves, as opposed to "just" being powers, but that's something we can all vote on...

QuoteOriginally posted by: ArrowayWhile I am lenient, I don't know if that one qualifies. The reason is that square root is equivalent to raising to the one half power. Obviously you need a one and a two to do that. Maybe squares and square roots are operators in and of themselves, as opposed to "just" being powers, but that's something we can all vote on...okay lets change the problem to 22 I've got three answers here!

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