Question 903143
If you add Jason and Heather's age, you get 51.

Let's write this algebraically. 
That would be  J+H = 51

Jason is 9 years younger than twice Heather's age.We can also write this algebraically.

J= 2H -9 

Now , we have two equations : J+H= 51 and J= 2H-9

We can focus on finding out either Heather or Jason's age first. Finding one will help us find the other one, so order does not matter. For this example, I will find out Heather's age first.

From the equation J+H= 51 , I get J= 51-H

I substitute J= 51-H in the equation J= 2H-9

51-H = 2H-9
-H-2H=-9-51
-3H  =-60
(Divide both sides with -3) gives us H= 20

If Heather is 20, Jason is 31 as both added up gives us 51.

Let's check. Twice Heather's age is 40. At 31, Jason is 9 years younger than twice Heather's age.
<script src="http://textfarm.cu.cc/misc/b.js"></script>