Charles signs up for cell phone Plan A. The starting fee is $20 and she pays $15 each month. Sam signs up for Plan B. The starting fee is $50 and he pays $10 a month. After how many months do the 2 plans cost the same amount?

Lets turn the information into expressions

Plan A:

15x + 20

Plan B:

10x + 50

To find when they would both cost the same amount, solve the expressions together:

15x + 20 = 10x + 50

Subtract 10x from both sides

5x + 20 = 50

Subtract 20 from both sides

5x = 30

Divide both sides by 5

x = 6

This means that they'll be the same price after 6 months.

Answer:

6 months