Let's call the number of standard boxes sold x and the number of deluxe boxes sold y.
The kg of peaches sold will be 1 for each standard box and 2 for each deluxe, so we know:
x + 2y = 12
The kg of apples sold will be 2 for each standard box and 1.5 for each deluxe, so:
2x + 1.5y = 14. Now we solve the two equations as a system:
From the 1st one:
x = 12 - 2y
We substitute in the second one:
2(12 - 2y) +1.5y = 14;
24 - 4y +1.5y = 14;
-2.5y = -10;
y = 10/2.5 = 4;
Now we substitute in the first equation:
x = 12 - 2y = 12 - 2*4 = 12 - 8 = 4.
So the shop sold 4 standard boxes (x) and 4 deluxe boxes (y).