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!

21-9=12, I hope I did it right.