The difference between two people buying two books is 8.7+6.3 = 15 yuan.

4.5 yuan buys 1 many books.

So the price of the book is 15+4.5 = 19.5 yuan.

Xiao Hua Dai 19.5-8.7 = 10.8 yuan.

Xiao Qiang brought 19.5-6.3 = 13.2 yuan.