LaKeisha is mowing lawns to raise money for a collector set of books. She charges $.10 for every square foot of lawn. The book set costs $150. If she has already mowed three 20 x 15 foot lawns, how many more square feet does she have to mow to earn enough for the book set?
She has mowed 900 square feet because 3 x 20 x 15 = <<3*20*15=900>>900
She has earned $90 because 900 x .1 = <<900*.1=90>>90
She has to earn $60 more dollars because 150 - 90 = <<150-90=60>>60
She has to mow 600 more square feet of lawns because 60 / .1 = <<60/.1=600>>600
#### 600