For this case we first define variables:
 x: bottles of apple juice
 y: bottles of orange juice
 We write the system of equations:
 x + y = 12
 1x + 1.75y = 15
 Solving the system of equations we have:
 x = 8
 y = 4
 Answer:
 you did buy:Â
 8 bottles of apple juice
 4 bottles of orange juice