Mrs.Kessler bought six books and four CD's. If each CD cost $10, how much did Mrs.Kessler pay for the CD's?

I am unsure why you mentioned the books, but I will attempt to answer this question.

$40

If there are 10 CDs in each book.

$600

Part A:

0 = the surface of the swimming pool

Part B:

Since the height of the slide is measured at +10 feet, then this means

that the top of the slide is 10 feet above the surface of the water (0) in the swimming pool.

Part C:

Since 3 feet of the pole was put in the water, then Kayla should mark the of the bottom of the pole as -3 feet.