The sum of two numbers is 21. the difference between the numbers is 9. find the numbers

For questions with 2 unknowns, we can solve it by using simultaneous equations, where we list 2 equations and solve the 2 unknowns. 

By the given info, we can list these 2 equations:
Let x and y be the two unknowns, where x is the larger number. 
x + y = 21      (1)
x - y = 9       (2)

we can solve simultaneous equations either by substitution or elimination. Today I'm using substitution method.

From (1),
we have to isolate an unknown.
x = 21 - y      (3)

Sub (3) into (2)
(21 -y) - y = 9
21 - 2y = 9
2y = 21 - 9
2y = 12
y = 6

Now, sub y = 6 to (1).
x + 6 = 21
x = 21 - 6
x = 15

Therefore, the answers are x=15 and y =6. You can double check any simultaneous equations by calculating both equations again with the numbers calculated!