Answer: 4 machines
Explanation:
In the given situation, the number of machines is inversely proportional to the time taken by them.
Given : 9 machines can fill an order in 20 hours.
Let n be the number of machines required to fill the same order in 45 hours.
Then , we have the following inverse variation equation:-
![n* 45=9*20\\\\\Rightarrow\ n=(9*20)/(45)=4](https://img.qammunity.org/2020/formulas/mathematics/high-school/a3xeomp3tqaj7fqcfa35r4giggqh0vovci.png)
Hence, the 4 machines would be needed to fill the same order in 45 hours.