I don't get inequalities

Question

A cell phone plan charges $39.90 a month, plus $0.05 per text message. Which expression can be solved to find how many text messages, x, can be spent while still keeping the monthly bill under $50?
A. 39.9+0.05x>50
B.x(39.9+5x<50
C.39.9+5x<50
D.39.9+0.05x<50
*please help I have a test tomorrow and I need to understand how to get the answer!!!*

Answer 1

The monthly charges start at $39.90.  They are never less than that.

There is an additional cost of $0.05 per text message.  The number of text messages is x.

 

The cost for these x messages is ($0.05/message)*(x messages)    [note: messages cancel out, leaving $)

So the complete expression for monthly cost is:

                 $39.90 + $0.05x

 

To keep the monthly cost under (less than) $50.00, we have the inequality:

                 $39.90 + $0.05x < $50.00      This is answer (D).

 

Now, review each of the other answers to determine what little difference made them wrong.