Chapter 3, Verses 8, 9 and 10 - Sun Tzu

It is the rule in war, if our forces are ten to the enemy's one, to surround him; if five to one, to attack him; if twice as numerous, to divide our army into two. If equally matched, we can offer battle; if slightly inferior in numbers, we can avoid the enemy; if quite unequal in every way, we can flee from him. Hence, though an obstinate fight may be made by a small force, in the end it must be captured by the larger force.
~ from The Art of War, Giles translation ~

Go here to read the introductory post to the verses of The Art of War.