Need to help my child with math, but I can't...

[Guest]

Claudia went to the movies, and expend 7.50 on the ticket, 3.50 on popcorn and ¼ of what she had left on a soda, she had left 6 dollars, how much does claudia took to the movies?

[Guest]

ticket + popcorn + soda + $6 = sum she had
ticket = $7.50
popcorn = $3.5
soda = ?

soda = 1/4 of (the sum she had - ticket - popcorn) = 1/4 * (sum she had - $11)

Suppose she had x dollars

7.5 + 3.5 + 1/4 * (x - 11) + 6= x
17 + 1/4 * (x - 11) = x

17 + x/4 - 11/4 = x
-3x/4 = 11/4 - 17 /multiply by 4 the both sides
-3x =- 57
x = 19

So Claudia had $19
She spent $19 - $6 = $13