Learn Finnish: Ravintolassa on kaksikymmentä pöytää ja seitsemänkymmentä tuolia. - There are twenty tables and seventy chairs in the restaurant.

You simply take the base number and add kymmentä (which is the partitive form of kymmenen, meaning "ten"). So, kaksi (two) + kymmentä = kaksikymmentä (twenty), and seitsemän (seven) + kymmentä = seitsemänkymmentä (seventy).

Why do we use pöytää and tuolia instead of their regular plural forms?

In Finnish, any number greater than one requires the noun following it to be in the partitive singular case. So after kaksikymmentä, you must use the partitive form pöytää (from pöytä), and after seitsemänkymmentä, you use the partitive form tuolia (from tuoli).

If there are many tables and chairs, why does the sentence use the singular verb on (is) instead of ovat (are)?

This is a "There is / There are" sentence, which in Finnish usually starts with a location (ravintolassa). In these types of sentences, the verb is always the singular on, no matter how many things there are. Furthermore, when a subject starts with a number greater than one in Finnish, the verb is always singular!

What does the -ssa ending on ravintolassa do?