Mrs. Green needs to store 21 math books on a shelf during school vacation. Each math book is 2.4 centimeters thick. If Mrs. Green stacks the math books on top of each other, how tall does the shelf have to be?

0.16 or 16%

step-by-step explanation:

lets take a coin for example.

if you toss it twice, it has a 1/4 chance of landing on heads both times because 1/2 * 1/2 is 1/4.

with the cards, the chance of picking dog is 0.4. having two people pick one is like picking a card twice in a row. so 0.4 * 0.4 is 0.16, or 16%