Final answer:
To find the total number of windows the cleaner can clean, multiply the number of windows per building by the number of buildings.
Step-by-step explanation:
To find the total number of windows the cleaner can clean, we need to multiply the number of windows per building by the number of buildings. Since the cleaner can clean 21 windows per office building,
we can use multiplication to find the total. Let's say there are 'x' buildings in total. So, the total number of windows the cleaner can clean is 21 times 'x', which can be represented as 21x.