Question

Gabriella's travel shampoo bottle holds 1/2 cup of shampoo. before leaving on vacation, she filled the bottle to the top with 1/8 cup of shampoo. How much shampoo was already in the bottle?​

I just need somebody to explain to me how I do this I don't even need the answer
I just need to know how to solve it

Answer 1

there was an already an amount in the bottle, and when 1/8 was added, the amount became 1/2

so

something + 1/8 = 1/2