Question

there were 845 million facebook users at the end of 2011, wich was up from 608 million users at the end of 2010.

51.5% of all users access facebook on mobile devices. How many users used mobile devices in 2011?

Answer 1

So,

All we have to do is multiply 845 million by 51.5%, or 0.515.
845 million * 0.515 = 435.175 million users

435.175 million, or 435,175,000 users accessed Facebook using mobile devices.