Comment
You need to set up a direct proportion. You have to relate Keith and Jared's ages to the ratio you were given.
Givens
Keith is 24
Keith / Jared = 3/5
J = Jared
Solution
3/5 = 24 / J Cross multiply
3*J = 5 * 24 Combine factors on the right.
3J = 120 Divide by 3
J = 120 / 3
J = 40
Conclusion
Jared is 40 years old.
