In a seating arrangement, 5 people (A, B, C, D, E) sit in a row. A and B must sit together, and C cannot sit at the ends. How many arrangements are possible?
60
This ensures they sit together. Now we have units: (AB), C, D, E → total 4 units.
Number of arrangements: 4! = 24 ways.
Within (AB), they can be (A, B) or (B, A), so 2 ways.
Without restrictions on C, total arrangements = 24 × 2 = 48.
C cannot be in positions 1 or 5. Number of ways to place C in middle positions = 3 choices.
After placing C, we arrange (AB) as a block + 2 other individuals in remaining 4 seats:
Arrangements = 3! × 2 = 6 × 2 = 12 ways.
3 choices for C × 12 arrangements = 36.
Total arrangements = 36, matching option (2).
For any natural number $k$, let $a_k = 3^k$. The smallest natural number $m$ for which \[ (a_1)^1 \times (a_2)^2 \times \dots \times (a_{20})^{20} \;<\; a_{21} \times a_{22} \times \dots \times a_{20+m} \] is: