premise: Two guys with helmets are on motorcycles, each with a woman riding behind him. hypothesis: There are four people on two motorcycles.
entailment: Guys and women are all types of people.  Each of the two guys is on a motorcycle, and each one has a woman riding behind him.  As there are two guys, each with one woman riding behind him, there are two women total; two women plus two guys equals four people total.  With motorcycles, riding behind someone, unless specified otherwise, refers to riding behind someone on the same motorcycle.  As there are only two guys, and since the women cannot be driving the motorcycles (because they are located behind the guys, out of reach of the motorcycles' controls), there cannot be more than two motorcycles, as driving a motorcycle requires a driver.  As "motorcycles" is plural, there must be at least two motorcycles; phrased alternatively, there cannot be fewer than two motorcycles.  As there cannot be more than two motorcycles, and there cannot be fewer than two motorcycles, there must be exactly two motorcycles.