Let's assume the initial length of the cloth is 100 units.
After the first cut, the length becomes: 100 - (80% of 100) = 100 - 80 = 20 units.
After the second cut, the length becomes: 20 - (90% of 20) = 20 - 18 = 2 units.
The decrease from the point after the first cut (20 units) to the point after the second cut (2 units) is 20 - 2 = 18 units.
To find the percentage decrease, we divide the decrease by the original value after the first cut and multiply by 100:
Percentage decrease =\(\left( \frac{18}{20} \right) \times 100 = 90\%\)
Since the value after the first cut is 20 units and it decreased to 2 units, which is half of 20 units, the percentage decrease is 50%.
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: