He needs to save up $400 because 4 x 100 = <<4*100=400>>400
He has 8 months to earn this money because 12 - 4 = <<12-4=8>>8
He needs to earn $50 a month because 400 / 8 = <<400/8=50>>50
He needs to do 5 tasks a month because 50 / 10 = <<50/10=5>>5
#### 5