Answer:
68
Explanation:
You want to know the number of marbles Chloe originally had in her collection if adding 36 marbles gave her a total of 104.
Equation
You can let a variable (o) represent the original number of marbles. You would choose this because the original number of marbles is what you're asked to find. It never hurts to use a variable name that reminds you of what it stands for.
The problem statement tells you that adding 36 to the original number gives a total of 104. As an equation, this looks like ...
o + 36 = 104 . . . . . . adding 36 to o gives 104
Solution
The properties of equality tell you that you can subtract the same number from both sides of the equation without changing the value of the variable.
Here, we want the variable by itself, so we want to get rid of the added number 36. We do that by subtracting 36 from both sides of the equation:
o +36 -36 = 104 -36 . . . . . subtract 36 from both sides
o = 68 . . . . . . . . . . simplify
This solution tells us that Chloe originally had 68 marbles.
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Additional comment
The way to solve this, or any, "word problem" is to read and understand the relationships the words are telling you, and what the words are asking for. Then write those relationships as math expressions, choosing variables and a procedure that will give you what the question is asking for.
Here, we could choose a variable that represents the quantity we want to find. That is not always the case.
Sometimes, you have to find an "intermediate" value that you need in order to be able to answer the ultimate question. You're not done when you find that intermediate value; you're only done when you are able to answer the question asked.