Answer
Plan A costs less when more than $300$ text messages are sent in a month.
Work Step by Step
Let $ x $ be the number of text messages in a month.
Better deal = smaller cost.
We want x for which
(Cost of plan A) $ \lt $ (Cost of plan B)
$ 0.08x+15 \lt 0.12x+3\qquad $ ... add $-0.08x-3$
$ 12 \lt 0.04x\qquad $ ... multiply with $25$
$300\lt x $
Plan A costs less when more than $300$ text messages are sent in a month.
