牛人来看SC 关于twice等数词用法 急

zlljolie

When drive-ins were at the height of their popularity in the late 1950s, some 4,000 existed in the United States, but today there are less than one-quarter that many
有个选项是. the number is less than one-quarter the amount
想问下牛人，这里的one-quarter到底是什么词性呢？
单从语法看，只看这个选项这句话the number is less than one-quarter the amount。这种表达语法正确吗？
以前遇到过twice，有twice+n. , twice+ what从句，这里的twice又是什么词性呢？
这种问题好纠结啊！