Distance covered by both buses together \(= \frac{180}{2} = 90 \text{ km}\)
Combined speed of both buses = 40 km/h + 60 km/h = 100 km/h
\(\text{Time taken} = \frac{\text{Distance}}{\text{Speed}} = \frac{90 \text{ km}}{100 \text{ km/h}} = 0.9 \text{ hours} \text{ or } 54 \text{ minutes}\)
Time taken by bus B to reach the destination:
Distance = 180 km
Speed = 60 km/h
\(\text{Time} = \frac{\text{Distance}}{\text{Speed}} = \frac{180 \text{ km}}{60 \text{ km/h}} = 3 \text{ hours}\)
Time taken by bus A to reach the destination:
Distance = 180 km
Speed = 40 km/h
\(\text{Time} = \frac{\text{Distance}}{\text{Speed}} = \frac{180 \text{ km}}{40 \text{ km/h}} = 4.5 \text{ hours}\)
Time waited by bus B at the destination:
Total time taken by bus A = Time taken for both buses to meet + Time waited by bus B + Time taken by bus B to return to the meeting point.
4.5 hours = 0.9 hours + Time waited + 3 hours
Time waited = 4.5 hours - 0.9 hours - 3 hours
= 0.6 hours or 36 minutes.
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: