How to write this problem as an equation

Question

the telephone company charges 50 cents for the first minute, 30 cents for each additional minutes and 70 cent service fee. The bill was $7.80. How long did the caller talk ?

 

I know the answer, but I am not sure how to write it as an equation.

Answer 1

If the phone company charges a service fee, then we know that no matter how many minutes the caller talks, they will always have to pay it. Because of this, we know our equation must include +0.70.

This is the tricky part, and you can write this equation two ways. I will show you one of the ways.

Except for the first minute, every minute is $0.30, so we can write it as .30x, where x is the number of minutes. The first minute is $.20 MORE than every other minute, so we can also add $.20 to the end of the equation.

 

Thus, we get: .30x + .90 = $7.80