You had helped me with a similar question.

Question

This one says :
A whole seller supplies 90 books while he was ordered to send 80 books by a retailer. If 15%trade discount was given, what was the net discount to the retailer?

I solved this one in the similar way to the one you helped me with but the answer doesn't match with the book. My book says answer is 27.5 % but I got 24.4%. There Was no answer for the one before this(the book had not printed the answer) for the previous question. Plzzzz help