Numbers above 10 are normally written in Western Arabic numerals for conciseness.
However, in this course we will use the traditional way of writing numbers with kanji, because this will give you more practice.
The way we count in Japanese resembles the way we count in English, but is more regular, as you can see when looking at the table below.
十一 zyuu iti
十九 zyuu kyuu
二十五 nizyuu go
四十八 yonzyuu hati
六十二 rokuzyuu ni
七十三 nanazyuu san
Note that forty is 四十 and not 四十,
and seventy is 七十 and not 七十.