47 is the sum of 19 and gail's score
47 = 19 + g
...
The key word here is "sum." Sum means we added two (or more) numbers together to come up with the sum, or the answer.
5 is the sum of 2 and 3.
5 = 2 + 3
So using that idea, 47 is the sum of 19 and another number.
47 = 19 + ?
We usually replace the question mark with a letter.
47 = 19 + x
I usually use "x" if there is only one thing to worry about. Oobleck chose "g," probably because it is Gail's score. Both ways are correct.
From there, can you solve it and see what x is? If not, let us know.
To find Gail's score, subtract 19 from 47.
Gail's score = 47 - 19 = 28