mathematics - How to make 21?

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.