Because of the vast number of commuters, a major problem facing modern megacities is:

Megacities have an impact on public health, like contamination of water. Overpopulated places can bring diseases and a lack of health protection.

Explanation:

Megacities are the base for gas emissions that could be very dangerous for the environment. Traffic jam is getting worse day by day. Almost half of the habitant of one city spends at least one hour in the car to get to work.

this is a good question! i'll do some research and come back when i have the answer!

the value of the 4 in the hundreds place is 400 while the value of th e 4 in the tens place is 40

400 is way bigger than 40.