Identify the percent of change as an increase or decrease. Then find the percent of change. Round to the nearest tenth of a percent, if necessary.
15 books to 21 books

40% increase

Step-by-step explanation:

from 15 to 21 is an increase

old value = 15

new value = 21

formula is:  (new value - old value) / old value x 100

[(21 - 15) / 15] x 100 = 6/15 x 100

= 2/5 x 100

= .4 x 100

= 40%

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%