She earned $42.5 on average per week because 425 / 10 = <<425/10=42.5>>42.5
She earned $22.5 each week from extra chores because 42.5 - 20 = <<42.5-20=22.5>>22.5
She did 15 extra chores a week because 22.5 / 1.5 = <<22.5/1.5=15>>15
#### 15