In order for mark to get to work he needs to drive 10 miles south and 20 miles east. how many miles would he have to go if there was a road going southeast straight to his destinantion?

For this case what you can do is see the problem as a rectangle triangle where you must find the hypotenuse of it.
 Therefore we have that the distance traveled will be:
 d = root ((10) ^ 2 + (20) ^ 2)
 d = 22.36 miles.
 
 if there was a road going southeast straight to his destinantion he would do to go 22.36 miles.

They are somewhat similar

The answer to your question is b