solution attached. please check if what I have done is right
Since all your numbers (i.e. the bases) are positive and not equal to 1, then they should be valid but there is one condition where if p = 0 and q = 0 then r = 0.
This is a special case because you will get r*(p+q) = 0 but p + q is 0 and so you get r*0 = 0 so you need a special condition that if p = 0 and q = 0 then r = 0 as well.
It's just a special case of division by 0 that you have to handle.