Geologists study the density of seismic activity in a region by comparing the number of earthquakes to the area of the region the seismic activity density of a region is found using the formula

3.0

Step-by-step explanation:

If the ratio is boys to girls, in fraction form (ratio form) that's

When we set up the ratio using the numbers, we have to keep the boy stuff with the boy stuff on top, and the girl stuff with the girl stuff on bottom.  So the ratio with the given numbers is

Reducing that down to decimal form, since 12 is divisible by 4 evenly, would be 3.0