Explain the answer.

Jackson, an unmarried taxpayer, qualified to itemize deductions. Jackson's adjusted gross income was $90,000 and he made a $20,000 cash donation directly to a needy family. During the year, Jackson also donated stock, valued at $30,000, to his church. Jackson had purchased the stock four months earlier for $15,000. What was the maximum amount of the charitable contribution allowable as an itemized deduction on Jackson's current year income tax return?