关於倍数的翻译:six times ____ it before 1990.填what 还是 that?...?

这里what=of that=of which,所以只要不限制字数，what和of that(或者of which)都可以填。

因为that或which引导定语从句，必须要有其修饰的对象，显然这里没有,所以不能直接用that或which。而用what则是宾语从句，直接这样用是可以的,相当于：Output in this company is now five times higher than what it was before 1990。
加of则形成介宾短语，相当于这样的句子: Red one is three times of blue one.=Red one is 2 times longer than blue one.

of that

直接填what 我们的期中考试题 今天才考的 给答案了