Ty number

Historical note: Large numbers of students fail to realise that “x-ty” in English means “x” tens.  This is a serious impediment to them learning clever mental strategising. In Old English numbers like sixty were “sixtig”.  The word “tig’” in those times meant ten.  As the English language has evolved it has lost the transparency for the meaning of sixty that it once had.
This a problem for many students.  This problem does not exist in Maori.  Hence the Curriculum suggestion that: “The Maori representations of 2-digit numbers are particularly useful for reinforcing place-value notions.  For example, “rua tekau ma wha” translates to “two tens and four.” (MiNZC page 37)

1. Ask  the students to get out 70 on the material they are using.  Ask how sets of ten there are. Watch for the students who go back and count 1, 2 , 3 , 4, 5, 6, 7; they have not picked up the code for tens in English.
2. Tell the students the code for “- ty”. It simply means tens and invite them to listen carefully for the presence of “-ty” at the end of a word.  For some students this will be important as they confuse say sixteen and sixty.  Ask how many tens in eighty by hearing the eight.
3. Repeat for ninety.
4. Practice both ways of saying and showing 60,70, 80 and 90. (60 is both six tens and sixty etc.)
5. Ask students to imagine sixty seven on material. How many tens needed? Repeat for many two digit numbers in the range 60 to 99. ( Lower than 60 runs into the problem of the irregularly named x-ty words
6. Approach the “fivety” problem. Students need to learn the “fif” in fifty is really a five. (Compare this “fiveteen’ and “ fiveth” in a race or as a fraction.). Read out some numbers from 50 to 59 and ask students to model them.
7. Approach “fourty” - it is not a problem orally but the spelling “forty” is a small concern. Read out some numbers from 40 to 49 and ask students to model them.
8. Approach “threety” - it is not a problem - students readily accept that “ thir” and three are the same thing. (Compare this with “threeteen’” and “ threeth”.) Read out some numbers from 30 to 39 and ask students to model them.
9. Approach “twoty”.  Twenty is a real problem for many students who have not picked up the clue that the “tw” in twenty is actually a two and the -ty is tens.  Read out some numbers from 20 to 29 and ask students to model them.