all important derivation of first book

In which of the following important features in a open book exam method?

find derivative of a first principle derivative cot x

A shopkeeper buys two books for Rs. 300. He sells the first book at a profit of 20% and the second book at a loss of 10%. What is the selling price of the first book if in the whole transaction there is no profit or no loss?

Some other derivation by first principal: