Answer
More than $300$ calling minutes in a month make plan A the better deal.
Work Step by Step
Let the number of calling minutes in a month be $x$.
Plan A monthly fees equal $\$15$ plus $\$0.08$ per minute.
Plan B monthly fees equal $\$3$ plus $\$0.12$ per minute.
In the mathematical form.
Plan A monthly fees $=15+0.08x$.
Plan B monthly fees $=3+0.12x$.
For plan A to be the better deal the inequality is.
Plan B monthly fees $>$ Plan A monthly fees.
The inequality is
$\Rightarrow 3+0.12x>15+0.08x$.
Add $-3-0.08x$ to both sides.
$\Rightarrow 3+0.12x-3-0.08x>15+0.08x-3-0.08x$.
Simplify.
$\Rightarrow 0.04x>12$.
Divide both sides by $0.04$.
$\Rightarrow \frac{0.04x}{0.04}>\frac{12}{0.04}$.
Simplify.
$\Rightarrow x>300$.
Hence, more than $300$ calling minutes in a month make plan A the better deal.
