Answer:
0.8 billion of photos on the site are selfies.
Step-by-step explanation:
This problem can be solved by a simple rule of three.
A study of 120,000 randomly selected photos posted to a social media site determined that 4% were "selfies," photos of oneself. This means that for 1 photo, 0.04 are selfies.
There are 20 billion photos, how many billions of photos on the site are selfies?
1 - 0.04
20 - x
0.8 billion of photos on the site are selfies.