Cost C=a/N where a is a constant of proportionality and N is the number of notebooks.
When C=0.60, N=16000 so a=0.60*16000=9600.
So C=9600/N. When N=12000, C=9600/12000=0.80.
Another way to do it is to use the fraction 16000/12000=16/12=4/3. We know that the cost increases as the number of notebooks decreases and vice versa. So we expect the cost for 12000 notebooks to be higher than 0.60, therefore we need a fraction that's greater than 1 to multiply by, i.e., 4/3 not 3/4. And 0.60*4/3=0.20*4=0.80.