Okay, here we have this:
Considering the provided information, we are going to calculate the requested cost of sending text messages, in dollars per text, so we obtain the following:
Then from the given information we obtain the following rule of three:
$12.45/83 messages=$x/1 message
($12.45/83 messages)/1 message=$ x
($12.45/83)=$ x
x=$0.15
Finally we obtain that the cost of sending text messages, in dollars per text is $0.15 per each message.