21. Use figures (even from 1 to 10) to express periods of time wheen they are used as technical measurements or significant statistics (as in discounts, interest rates, and credit terms).

a. a 35-hour workweek

b. a 30-year mortgage

c. a note due in 6 months

22. Spell out nontechnical references to periods of time unless the number requires more than two words.

a. a twenty-minute wait	c. twelve days from now	e. in twenty-four months	g. three hundred years ago
b. forty-odd years ago	d. eight hours later	f. two thousand years ago	h. But: 350 years ago

23. Centuries may be expressed as follows:

a. the 1900s	c. the twenty-first century
b. or: the nineteenth hundreds	d. the twentieth-century literature