Part A:
90.10 = 0.30x + 55
Part B:
Subtract 55 from both sides
90.10 = 0.30x + 55
-55 -55
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35.10 = 0.30x
Next, you need to isolate the variable so you must divide both sides by 0.30. You should get:
117 = x
Vin sent 117 text messages that month.
Part C: (this part is a bit unclear, though I will try to answer it as best I can)
I found the number of text messages Vin sent in the month of October by creating an equation that outlined his bill and solving it.