Let's start by calculating the cost of the Granny Smith apples.
Since the cost of each Granny Smith apple is $1.35 and Dahlia bought 6, we can calculate the total cost for the Granny Smith apples:
\[ \text{Cost of Granny Smith apples} = 1.35 \times 6 = 8.10 \text{ dollars} \]
Now, we know that the total cost of both types of apples was $15. Therefore, the cost of the Macintosh apples can be determined by subtracting the cost of the Granny Smith apples from the total cost:
\[ \text{Cost of Macintosh apples} = \text{Total cost} - \text{Cost of Granny Smith apples} \]
\[ \text{Cost of Macintosh apples} = 15 - 8.10 = 6.90 \text{ dollars} \]
Since Dahlia also bought 6 Macintosh apples, we can find the cost of each Macintosh apple by dividing the total cost of the Macintosh apples by the number of Macintosh apples:
\[ \text{Cost per Macintosh apple} = \frac{\text{Cost of Macintosh apples}}{6} \]
\[ \text{Cost per Macintosh apple} = \frac{6.90}{6} = 1.15 \text{ dollars} \]
Therefore, the cost of each Macintosh apple is $1.15.