Six plus four fifths a equals nine tenths a

6+4/5a=9/10a
make bottom numbers same
into 10
60/10+8/10a=9/10a
minus 8/10a from both sides
60/10=1/10a
multiply both sides by 10
60=a

If i think this is what your asking, he should look at the grams or pounds/ounces and compare the to their cost then max can identify which one is proportional and which is not ( i think xd)