Question

A bag contains ten identical blue marbles and ten identical green marbles. In how many distinguishable ways can five of these marbles be put in a row if there are at least two blue marbles in the row and every blue marble is next to at least one other blue marble

Answer 1

Answer: 11 different rows.

Explanation:

As the marbles are identical, we do not really care for permutations (as we can really not difference them)

If we have 5 blue marbles, we have only one combination.

B-B-B-B-B

If we have 4 blue marbles, we have 3 combinations:

B-B-B-B-G, G-B-B-B-B, B-B-G-B-B

This is because the blue marbles need to be next to another blue one, so from here we can do the same analysis.

If we have 3 of them, we have 3 combinations.

B-B-B-G-G, G-B-B-B-G, G-G-B-B-B

If we have 2 of them, we have 4 combinations

B-B-G-G-G, G-B-B-G-G, G-G-B-B-G, G-G-G-B-B

then we have 1 + 3 + 3 + 4 = 11 combinations