We know it charges $5 for 250 messages. Now we need to know how much 1 text is worth so we divide 250 divided by 250 = 1 and 5 divided by 250 = .02. Now that we know 1 text message is worth .02, you know have to mutiple .02 by 300 = 6$. So the answer is, they charge 6$ for 300 text messages.
