The sum of Jamie's age and Devon's age is 30. 
Ten years ago the product of their ages was 24. 
If Jamie is younger than Devon, what are their ages? 


J + D = 30
(J-10)(D-10) = 24

Solve that by substitution. That is

1. Solve the first equation for either letter.
2. Substitute the expression obtained for that letter 
   in the 2nd equation.
3. Solve the resulting equation for the remaining letter.
4. Substitute that in the equation obtained in step 1.
5. Solve for the other letter.

There are two solutions: (D,J) = (14,16), (D,J) = (16,14)
Since we are told that Jamie is younger, we discard the 
first. So the answer is Jamie is 14 and Devon is 16.

Checking:  The sum of their ages is 14+16 or 30.
Ten years ago, Jamie was 4 and Devon was 6, and 4×6 = 24.

Edwin