Деревянко, 5. Учебник. Unit 3. Lesson 2

It’s Olya’s copy-book. It’s Pat’s pencil case. It’s Max’s school bag.
It’s Bob’s pencil.

in — в

14 – fourteen
15 – fifteen
16 – sixteen
17 – seventeen
18 – eighteen
19 – nineteen
40 – forty
50 – fifty
60 – sixty
70 – seventy
80 – eighty
90 – ninety

Fourteen and six is twenty. Ten and forty is fifty. Fifteen and fifteen is thirty. Twenty and twenty is forty. Twenty and forty is sixty. Sixty and thirty is ninty.

[s] — (после глухих согласных) Jack’s, Pat’s. [z] — (после звонких согласных и гласных) Olya’s, Bob’s, [iz] — (после шипящих) Max’s.