Let's first find the total cost for the Granny Smith apples. Since Dahlia bought 6 Granny Smith apples at $1.35 each, we can calculate this as follows:
\[ \text{Cost of Granny Smith apples} = 6 \times 1.35 = 8.10 \]
Now, since the total cost for all apples (Granny Smith and Macintosh) is $15, we can find the total cost spent on Macintosh apples by subtracting the cost of the Granny Smith apples from the total cost:
\[ \text{Cost of Macintosh apples} = \text{Total cost} - \text{Cost of Granny Smith apples} = 15 - 8.10 = 6.90 \]
Next, 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:
\[ \text{Cost per Macintosh apple} = \frac{\text{Cost of Macintosh apples}}{6} = \frac{6.90}{6} = 1.15 \]
Thus, each Macintosh apple costs $1.15.