Final answer:
The number 6 is a solution to the inequality 21 + x < 28, as substituting x with 6 yields 27 < 28, which is true.
Step-by-step explanation:
The number 6 is a solution to the inequality 21 + x < 28. To find out if 6 is a solution, we can substitute x with 6:
Since 27 is less than 28, the statement is true, which means 6 is a solution to the inequality 21 + x < 28.