1. The problem states: "9 plus 10 is 21". We need to verify if this statement is true. 2. The operation involved is addition, which is combining two numbers to get their sum. 3. The formula for addition is $a + b = c$, where $a$ and $b$ are numbers and $c$ is their sum. 4. Substitute $a = 9$ and $b = 10$ into the formula: $$9 + 10 = 19$$ 5. The sum of 9 and 10 is 19, not 21. 6. Therefore, the statement "9 plus 10 is 21" is incorrect. The correct sum is 19.