Learn Farsi: ممکن است بیست درصد پاسخ‌دهندگان دیر برسند. - It is possible that twenty percent of the respondents arrive late.

You use the ezafe (the unseen 'e' sound). بیست درصد means "twenty percent", and it connects to the noun just like an adjective or possessor would: بیست درصدِ پاسخ‌دهندگان.

Why is the verb برسند in the plural, when "twenty percent" is technically a single amount?

When expressing percentages, the verb agreement depends on the logical subject. Since پاسخ‌دهندگان (respondents) refers to plural people, the verb must be plural. If the percentage referred to an uncountable or non-human noun, the verb would be singular.

Why is the verb برسند used instead of the indicative present tense می‌رسند?

The sentence starts with ممکن است (it is possible). Phrases expressing possibility or doubt require the following verb to be in the subjunctive mood. برسند is the present subjunctive form of "to arrive" for "they".

How is the word پاسخ‌دهندگان built?

It is a compound noun. پاسخ means "answer" and دهنده means "giver". The گان suffix is the plural ending used for animate nouns that end in a silent 'e' (like دهنده). Literally, it means "answer-givers".