A number below 10 is written out, unless it is used in an expression of time, measurement (length, volume, weight, velocity, inc.), or money:

two pages		eight attempts

31 employees		117 contracts

but

5 p.m.			8 cm

3 in.			$7 

Numbers of 10 or greater are written as numerals, which makes them easier to see and read. However, since a numeral stands out in text, nontechnical writers often write out numbers below 100 to avoid overemphasizing them.