Why would you use the associative property of multiplication to solve (10*4)*2?

So the associative property of multiplication states that you can multiply regardless of how the numbers are grouped, so the answer is you can multiply them in any order with out the result being wrong

Since the associative property says that the way you group three numbers when adding or multiplying doesn't change their sum or product, you could rearrange (10*4)*2 to be (2*4)*10 and still get the same product

Yes, two vertical lines can be complementary