What is a conversion factor?

How many centimeters are in 1 inch?

You have a block of metal with a mass of 2 kilograms. If the density of the metal is 8 grams per cubic centimeter, what is the volume of the block in cubic centimeters?

A car's fuel efficiency is rated at 25 miles per gallon. If 1 gallon equals 3.785 liters, how many kilometers per liter does the car achieve? (1 mile = 1.60934 km)

In a recipe, you need 2 cups of flour. If 1 cup equals 240 milliliters, how many milliliters of flour do you need?