Answer:
21.875%
Explanation:
We need an equation to represent this problem,
Let x be the number of books she had before giving away 42 of them,
x - 42 = 150
To find out how many books she had before turn the problem around and add 42 with 150,
150 + 42 = x
⇒ x = 192
Now that you know that divide 42 by 192,
Let n be the quotient of 42/192
42/192 = n
⇒ n = 0.21875
Now to turn that into a percent move the decimal 2 spaces to the left,
0.21875
⇒ 2.1875
⇒ 21.875%
Therefore there was a 21.875% change.