



 
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 $B.$ Make sure you check your answer,  


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