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!
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.
