This book is cheap enough for him to buy it.为什么要去掉it
问题描述：
This book is cheap enough for him to buy it.为什么要去掉it
答
因为这里的it和前面的this book重复了.
sth is cheap enough for sb to buy.固定词组,对某人而言,某件东西足够便宜,他有能力去买（有买东西的钱）
后面不需要再加it了.