Acell phone company charges $40 per month for unlimited calling, and $0.20 per text message sent. if f represents the number of text messages roxy sent last month, which expression represnts the cost of her bills last month before any taxes or additional fees?

The expression would be $0.20 x f. Once you figure out what the 'f' variable stands for, you have to multiply it by $0.20, and you will have your answer.

Okay Adiara- 

You have to pay $40 for the calling. So even if you don't text anyone (yeah right), you'd have to pay the $40. 

Now for those texts- $.10 each. But how many are you going to text? 5? 500? 50,000? We don't know just yet so we have to use a variable...let's call it t for texts. Each text costs $.10. In math, "each" means multiply. $.10 multiplied by the number of texts you send. 

Now we need to put those together- the $40 for calling and the $.10 * t for the texts. Put together...combine...ADD!

40 + .10t

Make sense?