all the windows of building can be cleaned by two people in nine hours
If two people can clean all the windows of a building in nine hours, it means that their combined work rate is 1/9 of the building's windows per hour.
Let's assume that each window can be cleaned at the same rate and that there are a total of N windows in the building. Therefore, in one hour, the two people can clean N/9 windows.
Now, if we want to determine the number of windows that can be cleaned by one person in nine hours, we can use the fact that their combined work rate is 1/9 of the building's windows per hour.
So, the work rate for one person is (1/9) windows per hour. Multiplying this rate by the number of hours, we get:
(1/9) windows/hour * 9 hours = 1 window.
Therefore, one person can clean one window in nine hours.