The classes in the pack cost $7.5 each because 75 / 10 = <<75/10=7.5>>7.5
An additional class costs $10 because 7.5 x (1 +(1/3)) = <<7.5*(1+(1/3))=10>>10
She buys 3 extra classes because 13 - 10 = <<13-10=3>>3
Her three additional classes cost $30 because 3 x 10 = <<3*10=30>>30
Her total cost is $105 because 75 + 30 = <<75+30=105>>105
#### 105