数词（Numeral）：表示数量或顺序的词叫做数词
数词分为基数词和序数词两类。
基数词（cardinal number）表示数量的多少，如：
There are 7 members in my family.
a, 基数词的表达方法，如：
seven 7 eighteen 18
eighty 80 twenty-one 21
three hundred and forty-five 345
b, 英语中的单位数词，如：
hundred 百 thousand 千
million 百万 billion 十亿
1， 000， 000， 000
billion million thousand
c, 基数词的读法（练习）：
268
1，369
2，684，651
4，968，752，353
序数词（Ordinal Number）表示排列的顺序，如：
The first time I saw him was yesterday.
第一 first
第二 second
第三 third
第四 fourth
第五 fifth
第六 sixth
第七 seventh
第八 eighth
第九 ninth
第十 tenth
第十一 eleventh
第十二 twelfth
第十三 thirteenth
第十四 fourteenth
第十五 fifteenth
第十六 sixteenth
第十七 seventeenth
第十八 eighteenth
第十九 nineteenth
第二十 twentieth
第二十一 twenty-first
第二十二 twenty-second
第二十三 twenty-fourth
第二十四 twenty-fourth
数词的其他用法
1. 小数的表达
小数点左边的数通常按基数词来读，若为三位以上的数，也可按编码式读法读出，即将数字单个读出；小数点右边的数通常按编码式读法单个读出。
6.86 six point eight six
14.15 fourteen point one five
345.456 three four five point four five six
345.456 three hundred and forty-five point four five six
0在小数中通常读作zero，也可读作字母o。如：
9.07 nine point o(zero) seven
2. 倍数的表达
用基数词加times表示，如：
Productivity has increased three times.
The girl is ten times cleverer than her brother.
Americans eat twice as much proteinas they actually need every day.
3. 百分数的表达（其中的百分号%读作percent）
6%读作：six percent
0.6%读作：zero point six percent
500%读作：five hundred percent
12.34%读作：twelve point three four percent
4. 分数的表达
a, 较小分数的一般读写方法。如：
1/3 one-third
2/3 two-thirds
3/5 three-fifths
b, 较复杂分数的简明读写方法。如：
22/9 twenty-two over nine
43/97 forty-three over ninety-seven
c, 整数与分数之间须用and连接。如：
四又二分之一four and a half
九又五分之二nine and two fifths
5. 数学运算的表达
6+5=11
Six plus five is eleven.
或 Six and five is eleven.
11-6=5
Eleven minus six is five.
或 Six from eleven is five.
4×5=20
Four multiplied by five is twenty.
或 Four times five is twenty.
20÷4=5
Twenty divided by four is five.
或 Four into twenty goes five.
15:5=3
The ratio of fifteen to five is three.
2的2次方是4 Two squared is four.
2的3次方是8 Two cubed is eight.
2的4次方是16 The fourth power of two is sixteen.
a > b a is more than b.
a < b a is less than b.
a ≈ b a approximately equals to b.
a ≠ b a is not equal to b.
