How do you get 24 with the numbers 3, 9, 7, and 1?

There is one way that you can do it that involves multiplication, division, addition, and subtraction, and that uses all the numbers only once.



Solving it using PEMDAS, you get:



So your answer is 

There are two answers to this.



But if you can only use addition, subtraction, multiplication, and division, you can also do:

The answer would be: b