Final answer:
Karl must include a benefit of $3,840 in his gross income.
Step-by-step explanation:
To calculate the amount Karl must include in his gross income, we need to determine the value of the groceries he purchased at 10% below Moe's cost. The cost to Moe was $6,400, so the cost to Karl would be $6,400 - ($6,400 * 0.1) = $5,760.
Karl spent $5,760 on groceries that were worth $9,600. Therefore, he received a benefit of $3,840 ($9,600 - $5,760). This benefit, known as an employee discount, must be included in Karl's gross income.