How to solve what I wrote on description

Question

How would I solve this problem?

cell phone company has two calling plans. plan a costs $20 per month plus 10 cents per minute of airtime. Plan B is only $10 per month but charges 20 cents per each additional minute. how many minutes would a customer need to talk per month for the two plans to cost the same.

Answer 1

If c=cost and m=minutes, then

 

Plan A: c=10m+20            Plan B: c=20m+10

 

Set both plans [same cost] equal to each other and solve for m

 

 10m + 20 = 20m + 10

         - 10            - 10

 10m + 10 = 20m

-10m           -10m

           10 = 10m

           10     10

 

            1 = m

 

So as long as they only talk 1 minute, it's equal! [I'd take plan A personally]