The mum tells Vera a joke:

После двадцати лет работы я уже так не думаю.

Let’s learn the numbers from 20 to 100!

Please remember that we say:

With the prepositions после/after, с/from, до/till we also use the Genitive case. We have already learned it in the Lesson 11.

Numbers also change their form when they are used in different cases!

In the Genitive case most of the numbers have the ending -и:
пяти, восьми, двадцати, шестидесяти.
The exceptions are 1 (одного), 2 (двух), 3 (трёх), 4 (четырёх), and also 40 (сорока), 90 (девяноста), 100 (ста), 1000 (тысячи).