How do you make $21$ from the numbers $1$, $5$, $6$, and $7$?
- You can use the operations of addition, subtraction, multiplication and division, as well as brackets.
- You must use each number exactly once.
- You cannot juxtapose numbers (i.e., 1 and 5 cannot be used as 15).
Answer
If this is an acceptable question, here is the answer:
$$\frac{6}{1-\frac{5}{7}}=21$$
This is a pretty well known problem that, while the math obviously works, is unintuitive enough that it is sometimes difficult to solve the first time you see it.
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