Well... 2/4+1/4=3/4
Or in other words, You spent 3/4 of your money (Half from Theater and 1/4 from the Arcade) and you have 1/4 of your money now
$17.50 is 1/4 of your money you had before you spent all the money.
To find the answer, you will multiply $17.50×4, since in this situation, you had 4/4 of money.
$17.50×4=$70.00
You had $70 before you spent your money
Hope this helped :D