19. what is the probability of tossing a penny and landing on heads two times in a row?
20. the area of one playing card is 24 square centimeters. 52 cars are stacked on top of each other to make a deck. what is the volume of that solid deck?

19). 
The probability of landing on heads the first time is 1/2.
The probability of landing on heads the second time is 1/2.
The probability of landing on heads both times is

                             (1/2) · (1/2)  =  1/4  =  25% .

20).
The volume of the solid deck is

               (length · width)  ·  (height)

           =          (24 cm²)   ·   (52 · the thickness of each car) .

A= r2 × 3.14 7x7 ↓ ↓ 49 × 3.14 = 153.86 → 153.9