#### Answer

Fill the blank with
$4.00+0.15x$

#### Work Step by Step

The monthly cost consists of two parts:
* the cost based on the number of messages,
** the fixed cost, regardless of the number of messages.
The sum of the two is the total cost.
So, total monthly cost equals
(number of messages)$\times$(cost of one)+ (fixed cost)
which we write as
$x\cdot 0.15+4.00$
or
$4.00+0.15x$
Fill the blank with
$4.00+0.15x$