Fun question!
How can we get a block of 3 consecutive performances such that the total block time is 30 min?-
Can we have 2 songs in a block of 3? No.
(5 + 5 = 10; we would need the last performance to be 20 min. No such option exists.)
-
Can we have 2 comedic monologues in a block of 3? No.
(10+10 = 20 min; while another comedic monologue can give us 30 min in this block, it is impossible for any other block of 3 to have a length of 30 min.)
-
Can we have 2 dance numbers in a block of 3? Again, No. (15+15 = 30 min; we hit the 30 min mark with only 2 performances. The next performance will take the block length above 30 min.)
Key insight: Every block of 3 consecutive performances will have exactly 1 dance number, 1 comedy monologue, and 1 song.What does this look like?Example -> Say, the first block is Dance-Song-Comedy. Then, the first block looks like this: (15-5-10)
The overall sequence
will look like this: 15-
5-10---15-5-
10---15-5-10
Observe --
Repeating the same sequence as Block 1 (Dance-Song-Comedy, in this case) is the
ONLY way to ensure that every
consecutive block of three has a 15, a 10, a 5.
For instance -> 15-10-
5--10-5-15--15-5-10. We can see that when we use a different sequence for block 2, we instantly break the "
30 min for any consecutive block of 3" condition.
Key Insight: The sequence of the first block IS the sequence for the other two blocks too. So, every block of 3 contains 1D, 1C, and 1S. And the sequence of D, S, and C, in the first block simply repeats for the second and third blocks.
Let's now find the number of different orders of arranging the 9 performances such that
every block of three consecutive performances is 30 minutes long 1) #ways of selecting 1D, 1C, and 1S for Block 1 = 3C1 x 3C1 x 3C1
2) Once selected, #ways to sequence the first block = 3!
3) #ways of selecting 1D, 1C, and 1S for Block 2 = 2C1 x 2C1 x 2C1
4) Once selected, #ways to sequence the first block = 1 (Because we must follow the same sequence as Block 1)
5) #ways of selecting 1D, 1C, and 1S for Block 3 = 1C1 x 1C1 x 1C1
6) Once selected, #ways to sequence the first block = 1 (Because we must follow the same sequence as Block 1)
Answer: 3 x 3 x 3 x 6 x 2 x 2 x 2 = 6 x 6 x 6 x 6 = 216 x 6 =
1296. Choice C. ---
Harsha