$$\frac{2}{5}$$ of a set of notebooks are sold on the first day. $$\frac{3}{4}$$ of the remaining got sold on the second day. If 75 notebooks remain still, how many notebooks were kept for sale?

let no of books be X

$$\frac{2}{5}th$$ were sold 

 $$X\times\frac{2}{5}th  = \frac{2X}{5}$$

ie $$\frac{2X}{5}$$ books were sold from X

remaining$$ X - \frac{2X}{5} = \frac{3X}{5}$$

from $$\frac{3X}{5}$$ books $$\frac{3}{4}$$ were sold

$$\frac{3X}{5}\times \frac{3}{4} =  \frac{9X}{20}$$

$$\frac{9X}{20}$$ books were sold from $$\frac{3X}{5}$$

remaining$$\frac{3X}{5} -\frac{9X}{20} =  \frac{3X}{20}$$ books

given $$\frac{3X}{20}$$ is equal to 75 books

$$\frac{3X}{20}= 75$$

X= 500