Question 637500
This may help your step 3.
Let x = number of red marbles to add to get 50%
You correctly calculated that there are 9 red marbles in the jar. The jar contains a total of 24 marbles is a given.
You want add x red marbles to the 9 to end up with (9+x) which is 50% or 1/2 the total number, which is now (24 + x) because you also add x to the total.
This results in the same ratio that you have, except you deduced that x = 6. That is you now have the algebraic equation
(9+x)/(24+x) = 1/2
Using cross multiplication we get
2*(9+x) = (24+x) or
18 + 2x = 24 + x
x = 24 - 18
x = 6
Answer: You need to add 6 red marbles.
Now you can rely on your math skill. However, don't toss out your super deductive reasoning.