I remember somewhere that you can use a rule of 10s to approximately estimate anything. So we would start by thinking how many windows are in all of Seattle? Well I think the population of Seattle is between 100000-1000000, so I'll decide around 1000000 as an overestimate. I think for each person there'll be between 1 and 10 windows so I'll decide 10. So now we have 10 million windows in Seattle approximately. I'll estimate that a bottle of generic window cleaner can last 1000 windows. 10000000/1000 = 10000. We'll need 10000 bottles of generic window cleaner. Now we just need to estimate a price per window cleaner. If we buy in bulk, we'll probably get a very good cheap price, so I'll underestimate that it'll cost $1 per window cleaner and towel. So it'll cost more or less $10000 to clean every window in Seattle, not counting hired workers.