Question 607291
Set your variables:

a = boxes of A
b = boxes of B
c = boxes of C

You will get three equations:

1: For N: a + 2b + 2c = 6
2: For P: 2a + b + 2c = 6
3: For K: 2a + 2b + c = 7

Solve using determinants, if you have studied them. Otherwise solve by elimination of variables:
Subtract the second equation from the first:
-a + b = 0
So a=b.
..
Substitute b = a into Eq. 1: 3a + 2c = 6
Substitute b = a into Eq. 3: 4a +  c = 7

Solve these together for a = 8/3; c = 3/5.