There are 12 months a year.关于这个句子中介词in省略的原因.....

Question

如果作为一个固定用法,能具体归纳总结下吗？

Answer 1

a有一项释义是“every”，所以只要能把“every”套进去的情况基本都不用介词了，如
I
work
five
days
a
(every)
week;
I
miss
her
a
thousand
times
a
(every)
day.