kind of mathematics extracurricular reading material that Mr. Zhang wants to buy can be bought in both bookstores A and B. Both bookstores are priced at 10 yuan each. However, the preferential condition of Bookstore A is that if you buy more than 10 books, you will get a 25% discount from the 11th book onwards. Bookstore B sells all books at a 15% discount. When Mr. Zhang wants to buy 20 books, which bookstore is cheaper to buy?
张老师要买的一种数学课外读物在甲、乙两个书店都可以买到．两个书店的标价都是每本10元．但是甲书店的优惠条件是购买10本以上，从第11本起七五折出售．乙书店是全部按八五折出售．张老师要买20本书时，到哪个书店买比较省钱？
<result>20</result>