Numbers:

1              Ichi

2              Ni

3              San

4              Shi    (chi)   or  Yon

5              Go

6              Rocku

7              Nana

8              Hatchi

9              Ku

10           Ju

11           Ju-ichi         (10 + 1)

12           Ju-ni            (10 + 2)

20           Ni-ju           (2  x  10)              21   =    Ni-ju-itchi      (2 x 10) + 1

30           San-ju        (3  x  10)               37   =   San-ju-nana     (3 x 10) + 7

40           Yon-ju       (4  x  10)

50           Go-ju        (5  x  10)