First, let's calculate the total cost of the Granny Smith apples. Dahlia bought 6 Granny Smith apples at a price of $1.35 each:
\[ \text{Cost of Granny Smith apples} = 6 \times 1.35 = 8.10 \]
Now, we know the total cost for both types of apples is $15. Therefore, to find the total cost of the Macintosh apples, we subtract the cost of the Granny Smith apples from the total cost:
\[ \text{Cost of Macintosh apples} = 15 - 8.10 = 6.90 \]
Dahlia also bought 6 Macintosh apples, so to find the cost of each Macintosh apple, we divide the total cost of the Macintosh apples by the number of Macintosh apples:
\[ \text{Cost of each Macintosh apple} = \frac{6.90}{6} = 1.15 \]
Thus, each Macintosh apple costs $1.15.