Sam, Sue, and Shelley formed a partnership. Sam received a 50 percent interest in the partnership in exchange for land with an adjusted basis to him of $30,000 and a fair market value of $50,000. Sue received a 25 percent interest in the partnership in exchange for $25,000 of cash. Shelley received a 25 percent interest in the partnership in exchange for $25,000 of cash. Six years after the date of contribution, the land contributed by Sam was sold by the partnership to an unrelated third party for $90,000. How much gain was required to be allocated to Sam as a result of the sale by the partnership?

a. $20,000

b. $30,000

c. $40,000

d. $60,000