Both crews will take 222.2 hours to complete the job.
No. The job won't be done before the cameras start rolling.
The crew that will take 400 hours does 1/400th of the total work required per hour.
The crew that will take 500 hours does 1/500th of the total work required per hour.
So if both crews work together, every hour will do 1/400 + 1/500.
Let's assume X is the number of hours. So let's make an equation and solve for X.
X/400 + X/500 = 1
500X/400 + 500X/500 = 500
500X/400 + X = 500
400*500X/400 + 400X = 400*500
500X + 400X = 200000
900X = 200000
X = 222.2222222
So both crews will take 222.222... hours to complete the job.
Now there's 7 days in a week and each day has 24 hours, so there's 7*24 = 168 hours in a week. And unfortunately, for the Mayor, that isn't enough time for both crews to be able to clean the streets.