Each tree A gives her six good oranges a month 10 x .6 = <<10*.6=6>>6
Each tree B gives her five good oranges a month because 15 x (1/3) = <<15*(1/3)=5>>5
On average, a tree gives her 5.5 oranges a month because .5(6) + .5(5) = 5.5
She has ten total trees because 55 / 5.5 = <<55/5.5=10>>10
#### 10