Answer
$0.15x + 4$
Work Step by Step
If we assume $x$ to be the amount of texts sent in a month, and we pay $0.15$ dollars per text, we can say that, at the end of the month, we pay $0.15x$. Since there is also an additional flat rate of $4$ dollars a month, we can conclude that the total paid for texts is: $$0.15x + 4$$
