Question

The sum of two numbers is x. If one of the numbers is 12, what is the other ?

Answer 1

If the sum of two numbers is x

Then the equation is like this:

a+b = x

Then it says one of the number is 12. Change either a or b into 12

12 + b =x

If we want to find the other number, we need the make it into the subject

b = x - 12

Hope that help.

Answer 2

 Y would be the other unknown number.
 
12 + Y = X

12-12+Y=X-12

Y=X-12