Ellen made 7 bouquets of the same size out of c red and d yellow roses. How many roses are in each bouquet?

the number of roses = C + D. The total has to be divisible by 7. If just one is then you will have roses left over. So C can be divisible by 7  and D can be divisible by 7. Or if you are careful, the total can be divisible by 7.

Take an example.
8 + 6 = 14 which will give you exactly 2 bouquets. Yet neither number is divisible by 7.

Or you could have 42 + 21 = 63 which would give you 7 bouquets of flowers. 

I'm making so many comments about this because there is no remainder and we have to explain why.

So the answer is 
(C + D)/7 is the number of possible bouquets <<<<< answer.