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) .