Will someone explain how i figure out number 9 and 10?

You dug a hole that was 8 feet deep. after taking a short break, you dug down 3 more feet in the same hole. when finished digging, a tractor accidentally filled the hole with 4 feet of dirt. how deep is your hole now?

What is the benefit of converting measures in a ratio to the same unit

For which value of x would this model make the least sense to use