A submarine is at the surface of the ocean, preparing to dive. It submerges and dives to feet. Then, it ascends feet to avoid hitting an undersea cliff. What integer represents the submarine's position in feet (depth) after it ascends to avoid the cliff?

Use b for boy and g for girl.

This is the resultant combinations:

b b b b
b b b g

b b g b
b b g g

b g b b
b g b g

b g g b
b g g g

g b b b
g b b g

g b g b
g b g g

g g b b
g g b g

g g g b
g g g g

If you count all the possibilities they are 16. Which is the same as 2^4 = 2*2*2*2, so I know that I didn't miss a combination.

The possibility of having no boys is one (g g g g combination) in 16, so the probability is 1 / 16 = 0,0625.