75% tax income or 75% tax incomes
100 billion dollars, which is 75% tax income(s?) for one yearThe whole sentence: my professor advances the theory by saying that a high-speed train system will cost 100 billion dollaors, which is 75% of tax income for one year.

Question

75% tax income or 75% tax incomes
100 billion dollars, which is 75% tax income(s?) for one yearThe whole sentence: my professor advances the theory by saying that a high-speed train system will cost 100 billion dollaors, which is 75% of tax income for one year.

Answer

"75% of tax income"
Each person/company/government has one income, though it may consist of more than one source, or may be specified in other ways e.g. annual income. 
In this case, the context is the government's income from taxation.

Answer

In the United States, in this kind of context, we would use the phrase "tax revenue," not "tax income." "The rail system would cost 75% of the city's annual tax revenue."

Answer

Thank you, I've updated my question with the complete sentence.

Answer

Context?
Neither seems right at first sight.
Can you give the rest of the sentehnce, or the sentence before and after?
.
We are asking for 100 billion dollars, which is 75% of the tax income for one year.

لا تفوّت فرصة تعلّم لغة جديدة وأنت مرتاح في منزلك. تصفّح مجموعتنا المختارة من مدرّسي اللغات ذوي الخبرة وسجّل في درسك الأول الآن!