Answer: it will take 5 months for both plans to cost the same.
Explanation:
Let x represent the number of months before the cost of the two phones and monthly service fees would be the same.
check sell offers a new phone for $50 and unlimited service for a monthly fee of $40. This means that the total cost of using this plan for x months would be
50 + 40x
cellular offers a new phone and unlimited service for a monthly fee of $50. This means that the total cost of using this plan for x months would be
50x
For both plans to cost the same, the number of months that it will take is
50 + 40x = 50x
50x - 40x = 50
10x = 50
x = 50/10 = 5