Rob is attending a party and was asked to bring 10bottles of soda. There are 4 different flavors available at the store, including grape soda which is Rob's favorite. If Rob wants to make sure to bring at least 3 bottles of grape soda, in how many different ways can he choose which sodas to bring to the party?

For this question, I don't get the solution. First, assume 3 soda is already in grape flavour, then only 7 soda need to be put into 4 different flavours. This means we only need to insert 3 bars. It didn't mention that there is at least one for each flavour, so it has 7+1 places to insert bars. So it should be 8 choose 3 right? But the solution says 10, so choose 3.

Even if it is like at least one for each flavour, it would be even smaller, 6 choose 3.

So why is it 10 choose 3 ????