Here I am on the last section of my "Permutations and Combinations" topic with this only one example dealing with an unique kind of problem as quoted below:

Now my problem quotes as follows:Example:Three letters are selected at random from the letters of the word BIOLOGY.

Find the total number of selections.

The answer isas you might expect.not

Because there are two letters O, you need to find the number of selections with

1)no letters O

2)one letter O

3)two letters O

and then add these together.

1)Number of selections with no letter O (ex. B, L and Y)

= number of ways to choosethreeletters from B, I, L, G, Y

= = 10

2)Number of selections with one letter O (ex. O, B and L)

= number of ways to choosetwoletters from B, I, L, G, Y

= = 10

3)Number of selections with two letters O (ex. O, O and B)

= number of ways to chooseoneletter from B, I, L, G, Y

= 5

Therefore, total number of selections = 10 + 10 + 5 =25

In a mixed pack of coloured light bulbs there arethree redbulbs,one yellowbulb,one bluebulb andone greenbulb.

Four bulbs are selected at random from the pack.

How many different selections are possible?

Ans: 7

That means if I represent all the 6 bulbs with their corresponding color letters as in the set below,

How manycombinationsof 4 letters from the 6 letters, where 3 letters R is repeated, can I get?

{R R R Y B G}

But I still have trouble applying the example solution there . I need help please ! Thanks.