Answer:
Cost of one New York style cheesecake = $5
Cost of one apple cheesecake = $9
Step-by-step explanation:
Let cost of one New York style cheesecake be = 
Let cost of one apple cheesecake be = 
Given:
3 New York stile cheesecake and 7 apple cheese cake were sold for a total of $78.
Cost of 3 New York style cheesecake be = 
Cost of 7 apple cheesecake be = 
Total = $78
We have:
A) 
12 New York stile cheesecake and 12 apple cheese cake were sold for a total of $168.
Cost of 12 New York style cheesecake be = 
Cost of 12 apple cheesecake be = 
Total = $168
We have:
B) 
Dividing each term in equation B by 12.


Solving for
in terms of 
Subtracting
both sides.


Substituting
in equation A.

Using distribution:


Subtracting both sides by 42.


Dividing both sides by 4.

∴ 
Substituting
in 
∴ 
∴ Cost of one New York style cheesecake = $5
∴ Cost of one apple cheesecake = $9