Systems of Equations · Hard problem

Plan A costs $\$30$ a month plus $\$0.05$ per minute of calls. Plan B costs $\$40$ a month plus $\$0.02$ per minute.

(a) At how many minutes do the two plans cost the same?   (b) What is that cost?   (c) Which plan is cheaper for someone who talks for twice that many minutes — enter A or B?