Roman numerals

 The roman numerals are made of 7 basic symbols

I - 1                                                              

V- 5

X - 10

L - 50

C - 100

D - 500

M - 1000

    These are roman numerals from 1 -30

I -1               XI - 11            XXI- 21

II - 2             XII - 12          XXII - 22

III - 3            XIII - 13        XXIII - 23

IV - 4            XIV - 14         XXIV - 24

V - 5              XV - 15          XXV - 25

VI - 6              XVI - 16        XXVI - 26

VII - 7            XVII - 17      XXVII - 27

VIII - 8           XVIII - 18     XXVIII - 28

IX - 9              XIX - 19       XXIX - 29

X - 10             XX- 20           XXX - 30