$50 per phone
Work Step by Step
$(500+20x)(75-x)$ $37500-500x+1500x-20x^2$ $-20x^2+1000x+37500$ $-b/2a = -1000/2*-20$ $-b/2a = -1000/-40 = 25$ $x=25$ $75-25=50$
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