A man purchases 100 copies of a book from the publisher and gets a discount of 25%. He buys 50 copies from a retailer at a discount of 10%. He got an overall discount of:

Let the marked price of the 1 book =x Rs.

The price of the book of 100 copies =100x

Price of the books of 100 copies after the discount $$=\dfrac{75 \times 100x}{100}=75x$$

Price of the book of 50 copies before discount =50x

Price of the book of 50 copies after 10% discount $$=\dfrac{90\times 50x}{100}=45x$$

Hence net price of 150 copies of the book after the discount =75x+45x=120x$$

The marked price of the book =150x

Hence the discount $$=\dfrac{(150x-120x)\times 100}{150x}=\dfrac{30x \times 100}{150x}=20\%$$