voodoochild wrote:
A bag has 4 blue, 3 yellow and 2 green balls. The balls of the same color are identical. In how many
ways can a child picks ball(s) out of the bag? He could even decide to pick ZERO balls.
A. 60
B. 1260
C. 24
D. 120
E. 9
JeffTargetTestPrep does this explanation make sense?
The distinction here is between the number of selections of "identical things into identical groups" and the number of ways to select "identical things into identical groups" (i.e. that all the balls of the same color are the same and the groups they are put in are the same, so we are not at all concerned with whether we select Blue 1, Blue 2, Blue 3, or Blue 4 nor are we concerned with the group we put them in comparison with others (if we put B1 and G1 together, or G2 and B2 it all counts as the same thing). If either of these things were considered the number would be much larger.
We instead say that the ways to select Blue are from 0 to 4, inclusive ... (4-0)+1 = 5 ways to select a blue ball.
Meaning that we can choose 0 to 4 blue balls, regardless of what else is going on (because again, we don't care about the relation to other blue balls or other balls in the group).
We multiply 5*4*3 = 60 because that's the number of WAYS we can put balls together, below are all the possibilities:
9 WAYS TO CHOOSE 1 TYPEB, BB, BBB, BBBB,
G, GG
Y, YY, YYY
26 WAYS TO CHOOSE 2 TYPESBG, BBG, BBBG, BBBBG,
BGG, BBGG, BBBGG, BBBBGG,
BY, BBY, BBBY, BBBBY,
BYY, BBYY, BBBYY, BBBBYY,
BYYY, BBYYY, BBBYYY, BBBBYYY,
YG, YGG,
YYG, YYGG
YYYGG, YYYGG
24 WAYS TO CHOOSE 3 TYPESBYG, BBYG, BBBYG, BBBBYG,
BYYG, BBYYG, BBBYYG, BBBBYYG,
BYYYG, BBYYYG, BBBYYYG, BBBBYYYG,
BYGG, BBYGG, BBBYGG, BBBBYGG,
BYYGG, BBYYGG, BBBYYGG, BBBBYYGG,
BYYYGG, BBYYYGG, BBBYYYGG, BBBBYYYGG,
That adds up to 59, what's left is
1 WAY TO CHOOSE 0 TYPES so + 1 = 60.