A certain race is made up of three stretches: A, B and C, each 2 km long, and to be covered by a certain mode of transport. The following table gives these modes of transport for the stretches, and the minimum and maximum possible speeds (in km/hr) over these stretches. The speed over a particular stretch is assumed to be constant. The previous record for the race is 10 minutes.
| Stretch | Mode of Transport | Min. Speed ($\mathrm{km/hr}$) | Max. Speed ($\mathrm{km/hr}$) |
|---|---|---|---|
| A | Car | 40 | 60 |
| B | Motorcycle | 30 | 50 |
| C | Bicycle | 10 | 20 |
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: