To find the unit price of a pound of apples, we start with the cost of the 3-pound bag:
\[ \text{Unit price} = \frac{\text{Total cost}}{\text{Weight in pounds}} = \frac{5.25}{3} = 1.75 \text{ dollars per pound} \]
So, the unit price of a pound of apples is $1.75 per pound.
Next, to find out how many pounds of apples can be purchased for $17.50, we use the unit price:
\[ \text{Number of pounds} = \frac{\text{Total money}}{\text{Unit price}} = \frac{17.50}{1.75} = 10 \text{ pounds} \]
At this rate, 10 pounds of apples can be purchased for $17.50.
Lastly, we can express the direct variation between the total cost \( y \) and the number of pounds of apples \( x \) with the equation:
\[ y = kx \]
Where \( k \) is the unit price. Since the unit price is $1.75, the equation becomes:
\[ y = 1.75x \]
So, the equation that represents the direct variation is \( y = 1.75x \).