Question 483339
If I understand this correctly, this one is fairly simple. 
If C=the total cost and t=the months you have the phone, you just need to put those things into an equation using the number values. $20 a month for the plan and a one-time, flat fee of $40 for the phone itself.
C=20t+40
So, if you had and operated the phone for 5 months, it would be C= 20(5)+40
C=100+40, The cost at this point would be $140.

Hope I helped! :)
-Rosey