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

# Answer

600
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
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?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
600
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?
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