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 = 
<gadget id="calculator">3*20*15</gadget>
<output>900</output>
900.
She has earned $90 because 900 x .1 = 
<gadget id="calculator">900*.1</gadget>
<output>90</output>
90.
She has to earn $60 more dollars because 150 - 90 = 
<gadget id="calculator">150-90</gadget>
<output>60</output>
60.
She has to mow 600 more square feet of lawns because 60 / .1 = 
<gadget id="calculator">60/.1</gadget>
<output>600</output>
600.

<result>600</result>