I got 1/3. Here is what I did.
First, to break even, Caramel coffee's contribution should make up for the lost profits due to cannibalisation. Therefore, I calculate profits lost due to cannibalisation.
total volume (before new coffee flavour) is 60m+20m=80m
30% of 80m is 24m, split as follows: Plain 18m, Hazelnut 6m (in proportion to their sales)
Therefore profits lost are 18m*$0.5/box+6m*$0.3/box=$10.8 million
to compensate for these lost profits, I need to sell $10.8/0.3=36m boxes of the Caramel flavour.
The 'new sales' are 36-(18+6)=12 m boxes which is 1/3 of 36. OK?