Gregory McDonald Sold a Piece of Land He Purchased for $5,000

Gregory McDonald sold a piece of land he purchased for $5,000 many years ago. He recently mortgaged the property for $30,000, but after six months he sold the land for $50,000 to his cousin, Gus.

A) If Gus purchased the land for cash and Gregory paid off the mortgage, what is the contract price, the gross profit, and the gross profit percentage?
B) If Gus took the land subject to the mortgage and paid $5,000 a year, plus 10 percent interest for four years, starting with the year of sale, what is the contract price and the gain in the year of the sale?