Problem: Total flavor should not change in a strong interaction. Confirm that the decay of the eta-c does not change the total flavor or electric charge.
Hint: Just define flavor quantum number (c, -c) where c is the quark and -c is the antiquark, for the two quarks in eta-c. This sum is zero. Do the same with the flavor quantum numbers of all the out going particles; they must add up to zero again. Also eta-c is neutral.
A picture the decay of an eta-c particle.
Quarks