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 Question from Z, a student: How do I find how many books are in a library given a problem like this... The number of books increase after buying 20% more in May. The number increased again by 20% after they were given 300 books. How many books did it start with?.

Hi,

20% is $\large \frac{2}{100} \normalsize = 0.20$ so 20% of a number is $0.20$ times the number.

Suppose the library started with $B$ books. In May they bought 20% more books so they bought $0.20 \times B$ books. Now they had

$B + 0.20 \times B = 1.20 \times B \mbox{ books.}$

20% of this number is 300. Solve for $B.$