You are looking for the GCF or greatest common factor of 15 and 20 a way to do the is the upside down cake method.
Answer:
-8
Step-by-step explanation:
There are two ways to find the total with tax
Either you can do
cost + cost x tax rate
or
cost x (1+tax rate)
Since the second one requires less steps, it's easier to do that.

The cost with tax is
$17.27
Answer:
31 in sq
Step-by-step explanation:
The interior shape is inside the exterior shape
first get the area of the big shape
9 times 15 = 135 in sq
now get the area of the interior shape:
8 times 13 = 104 in sq
now subtract interior shape from exterior
135 - 104 = 31 in sq
that's the shaded part