Two-Digit Numbers
How old are you? How many stairs are in your house? How many sweets in the packet? Once numbers
grow past nine, we need two digits to write them — and they turn up everywhere:
the pages of a book, the door numbers on a street, the age on a birthday badge.
Once you know place value —
that a two-digit number is just some tens and some ones —
reading and writing those numbers becomes easy. The tens digit and the ones
digit, side by side, give the number its name.
Take 34. The first digit, 3, sits in the
tens place, so it means three tens — that's thirty. The second digit,
4, sits in the ones place, so it means four ones.
Put them together:
34 = 3 \text{ tens} + 4 \text{ ones} = 30 + 4 = \text{“thirty-four”}
That middle step, 30 + 4, is called the expanded form:
it pulls the number apart and shows you exactly what each digit is worth. Say the tens part first,
then the ones part — thirty, then four — "thirty-four". Most two-digit names
work in just this way.
See it: tens-rods and ones-cubes
Here is the same idea you can count. A ten is drawn as a tall rod made
of ten little cubes stuck together, and a one is a single loose cube. Count the
rods first (those are the tens), then count the loose cubes (those are the ones), and you have
read the number. Press Refresh to build a brand-new one and try again.
The tens words
The tens words follow a tidy -ty pattern: twenty,
thirty, forty, fifty, sixty,
seventy, eighty, ninety. Each one is simply
that many tens — 50 is "fifty", which is five tens and no ones,
or 50 + 0.
The teens are the tricky ones. From 13 to
19 we don't say "ten-three" or "ten-four" — we say
thirteen, fourteen, fifteen, and so on, with the ones part spoken
first. And 11 and 12 have their own
special names, "eleven" and "twelve", which don't sound like ten-and-something at all. So the
teens are worth learning by heart.
In a teen like 16, the ones come out of your mouth first —
"six-teen" — even though the ten is really the bigger part! In every other two-digit
number ("thirty-four", "fifty-one") we say the tens first. English just kept some very old
words for the teens, so they break the pattern. The wise owl's advice: don't try to work the
teens out from the digits — simply memorise them, the way you memorised "eleven".
Press play. We build a two-digit number from its tens column and its ones column, then read
its name aloud. Replay it: a different number is built each time, so you can practise naming
each one.
A few worked examples
Every two-digit number splits the same way. Read the tens digit, then the ones digit:
- 27 is 2 tens and 7 ones —
20 + 7 — "twenty-seven".
- 61 is 6 tens and 1 one —
60 + 1 — "sixty-one".
- 90 is 9 tens and 0 ones —
90 + 0 — "ninety". The 0 says "no ones", which keeps the 9 in the
tens place.
The biggest two-digit trap is reading a digit at its face value instead of its place value:
- The 2 in 23 does not
mean "two" — it means twenty, because it sits in the tens place.
- In 37 the 3 is 3 tens
(thirty), so 37 is "thirty-seven", not "three-seven".
- "Forty" is spelled with no u — forty, even though "four"
has one. It is still just four tens.
The most tens you can have is nine (ten tens would bundle up into a hundred), and the
most ones is nine too. So the biggest two-digit number is 9 tens and
9 ones — 99, "ninety-nine". Blow it up by
just one more and it pops over to 100: a brand-new hundreds
column begins, and we need three digits.
The tens digit of 24 already answers it: 2.
You can fill 2 full bags of ten cookies, and 4
cookies — the ones digit — are left loose on the plate. That is exactly what
"24 = 2 tens and 4 ones" is telling you.
The two digits of a number are a packing list: how many tens-bags, and how many singles.