has been to the zoo for five times 和My mom has been to Beijing twice.为什么一个要用for一个又不用?

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has been to the zoo for five times 和My mom has been to Beijing twice.为什么一个要用for一个又不用?
has been to the zoo for five times 和My mom has been to Beijing twice.
为什么一个要用for一个又不用?

has been to the zoo for five times 和My mom has been to Beijing twice.为什么一个要用for一个又不用?
twice这个词本身就是adv.两次;两倍;兼
for+n.for two times=twice