x = total value of merchandise sold (in dollars)
y = take home pay (in dollars)
Job A: y = 0.15x + 750
Job B: y = 0.20x + 675
We want to know when job B's pay will be larger than job A's pay. So set the expression for job B larger than job A, solve for x
0.20x + 675 > 0.15x + 750
0.20x - 0.15x > 750 - 675
0.05x > 75
x > 75/0.05
x > 1500
Answer: Miranda must sell more than $1500 worth of merchandise to have job B pay more than job A.
