Some countries that span both Europe and Asia include Turkey, Russia and Kazakhstan. They are called trans continental countries. Even though Europe and Asia are continents in their own rites, geographically speaking they span one landmass known as Eurasia. The landmass rests on the tectonic plate known as the Eurasian plate.
1/2 * 20 = 10
You can also think of it as the 20 being divided by two.
human evolution started in (what ever area Africa was during Pangea) then threw what was connecting the continents (land bridge that connected Alaska from {don't quote I'm unsure cause I might have this backwards Antarctica} into what would become the Americas, to warmer climate.