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Mathematics · Year 1 · Chapter 1

Tens and ones

Practice

14: fourteen — 1 ten and 4 ones
seventeen: 17 — 1 ten and 7 ones
13: thirteen — NOT thirty
16: sixteen — ten and six
nineteen: 19 — 1 ten and 9 ones
11: eleven — 1 ten and 1 one
15: fifteen — 1 ten and 5 ones
twelve: 12 — 1 ten and 2 ones
18: eighteen — ten and eight
20: twenty — 2 tens and 0 ones

Easy questions

17 = 1 ten and ? ones: 7
fourteen — write the numeral: 14
13 in words: thirteen
1 ten and 2 ones: 12
nineteen — numeral?: 19

Medium questions

30 or 13 — which one is thirteen?: 13 — '-teen' means ten-and-some
1 ten and 0 ones: 10
sixteen = ten and ?: six
Which is more: 19 or 21?: 21 — 2 tens beats 1 ten
Is 15 backwards (51) the same number?: No — 15 is 1 ten 5 ones; 51 is 5 tens 1 one

Hard questions

2 tens and 3 ones: 23
Ten more than 7: 17
Which teen number has 8 ones?: 18 — ten and eight
Is 20 a teen number?: No — it's exactly 2 tens, nothing extra
Sam writes 'fourteen' as 41. What did he mix up?: The order — we say ones first (four-teen) but write the ten first: 14

Lesson

Teen numbers are ten-and-some

In a two-digit number, the digits have jobs. The left digit counts TENS, the right digit counts ONES. So 14 is 1 ten and 4 ones — ten and four more. Every teen number is a ten with some extra: thirteen is ten-and-three, seventeen is ten-and-seven. The catch is English: we SAY the ones first ('four-teen') but WRITE the ten first (14). That mismatch is why 14 turns into 41 under tired hands, and why thirteen and thirty get swapped. The endings tell them apart: '-teen' means ten-and-some (13), '-ty' means that many whole tens (30). When in doubt, ask the digits what their job is.

14 = 1 ten and 4 ones. 41 = 4 tens and 1 one. Very different amounts of eggs.
thirteen → 13 (ten and three). thirty → 30 (three tens).
17 = ten and seven. Say it as 'ten and seven' and the writing order makes sense.
19 is one more than 18, one less than 20 — two tens exactly.

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