Question 251836
Plan a:
100+.40(x-100)
100+.40x-40
.40x+60
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Plan b:
250+.25(x-250)
250+.25x-62.50
.25x+187.5
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.25x+187.5<.40x+60
.25x-.40x<60-187.5
-.15x<-127.5
x>-127.5/-.15 [note: the > sign changes to a < sign when dividing by a negative value.
x>$850 any bill greater than $850, plan B will be the best buy.
Proof:
Let the bill be $900 
.25*900+187.5<.40*900+60
225+187.5<360+60
412.5<420