Final answer:
The cost of 6 pounds of apples is $7.80, calculated by first determining the cost per pound ($1.30) based on 3 pounds costing $3.90 and then multiplying the cost per pound by 6.
Step-by-step explanation:
The cost of apples varies directly with the number of pounds purchased, which tells us that the cost per pound is a constant rate. Since 3 pounds of apples cost $3.90, to find the cost of 1 pound, we divide the total cost by the number of pounds, resulting in $3.90 ÷ 3 pounds = $1.30 per pound. Knowing this rate, we can now calculate the cost of 6 pounds of apples by multiplying the cost per pound by the number of pounds: $1.30 per pound × 6 pounds = $7.80. Therefore, 6 pounds of apples would cost $7.80.