Answer: The diver is 23 feet under sea level
Explanation:
If we draw a vertical number line (image attached), where point 0 is sea level (the diver's initial position), the first point under sea level is -20, since we are told it dives a distance of 20 ft (the negative sign is only to indicate the direction, since a distance cannot be negative).
Then, the diver goes 10 ft deep. This means now it is 30 ft under sea level and the second point is -30.
After that the diver swims up 12 ft, hence we have to count 12 points up in our number line and the corresponding point is -18.
Finally, the diver swims 5 ft lower and keep that position. Hence, we have to count 5 points below point -18 in the number line, resulting in the last point: -23.
This means the diver's final position is 23 feet under sea level.
If we want to prove it with numbers, we can do it as follows:
20 ft (down)+10 ft (down)-12 ft (up)+5 ft (down)=23 ft