Your numbers are stated in terms of miles. Greenville is a total of 120 miles from Rick. He drives x number of miles and sees that there is still 20 miles to go, so at the end of the driving, he has driven the unknown number of miles, x plus the 20 to get where he's going. That expression is x + 20. We set that equal to the total number of miles he has to drive and we get the expression for the trip to be x + 20 = 120, option C.
