The number of ways in which any four letters can be selected from the word “CORGOO’ is?

Since there are 3 Os and 3 other letters, the selection of 4 letters can be done in three possible ways: one O, two Os and three Os.
One O: Number of ways of selecting 4 letters is 1.
Two Os: Number of ways of selecting 2 of the other 3 letters is $$^3C_2$$ = 3 ways
Three Os: Number of ways of selecting 1 of the other three letters is $$^3C_1$$ = 3 ways
So, total number of ways of selection = 3 + 3 + 1 = 7 ways