He was president of South Africa until he stepped down in the year 199919991999.Let xxx represent any year. Write an inequality in terms of xxx and 199919991999 that is true only for values of xxx that represent years after the year that Nelson Mandela stepped down from his presidency.

Answer: x > 1999

An inequality is a relation that states that two quantities are not equal. 
The inequality can be expressed with different symbols, with different meanings:
a ≠ b means that the two values are not equal, but it doesn’t compare them in size
a > b means that a is strictly greater than b
a < b means that a is strictly less than b
a ≥ b means that a is greater than or equal to b
a ≤ b means that a is less than or equal to b

Given:
x = any year and
1999 = year in which Mandela stepped down from his presidency,
the question asks for an inequality that takes into account any year after 1999, which means any year greater than 1999.

Writing this sentence with mathematical symbols it would be x > 1999