Answer:
They will both have paid the same amount after 4 months
Explanation:
Assume that they will pay the same amount after x months
∵ Billy pays $30 for his new phone
∵ He only pays $12.50 each month
→ Find how much he will pay in x month
∴ He will pay → 30 + 12.50x
∵ Annie got her new phone for free
∵ She only pays $20 a month
→ Find how much she will pay in x month
∴ She will pay → 20x
∵ They will pay the same amount
→ Equate their expressions above
∴ 20x = 30 + 12.50x
→ Subtract 12.50x from both sides
∵ 20x - 12.50x = 30 + 12.50x - 12.50x
∴ 7.50x = 30
→ Divide both sides by 7.50
∴ x = 4
∴ They will both have paid the same amount after 4 months