Final answer:
The question relates to constructing a cost equation for text messages and calculating the monthly cost for sending 125 text messages. However, without specific cost details, the exact equation and total cost cannot be determined.
Step-by-step explanation:
The student's question appears to be related to writing an equation based on a per-message cost structure and calculating the total monthly cost given the number of text messages sent. However, there seems to be some vital information missing, like the cost per message or a base fee, which would be necessary to construct the equation and calculate the cost for 125 text messages. Using the provided information from Practice Test 4 Solutions 12.1, a general example equation form would be c = mt + b, where c is the total monthly cost, t is the number of text messages, m is the cost per message, and b is the base fee or monthly charge.
Without the specific cost details for text messages, we cannot provide an exact answer for part B of the question and determine the total monthly cost for 125 text messages.