Hello! So in order to solve this problem, it's best to start off making an equation. $18 + $.15x = $29.25
In this equation x represents the number of minutes she used. So now, we just have to solve for x.
First subtract $18 from both sides. Your new equation will be $.15x = $11.25. Then divide both sides by $.15 to get x by itself. $11.25/$.15 = 75.
So we can conclude that Bella spoke on the phone for 75 minutes this last month. Hope this helps!