Do you think its possible to reach 30mph on a flat level street on a 21 speed mountain bike?

Sure it's possible. Highly unlikely for a kid on a dept store bike, but possible. Assuming default values, which a 21-speed mountain bike certainly can't match, it calculates out to almost 900 watts - well over 1 horsepower. That's to hold the speed; it would take more to accelerate *to* the speed.

Depends on how strong the rider is. A 21 speed mountain bike will be fairly heavy and low quality, not the best sort of bike for trying to go fast. A strong professional rider might get up past 30 mph for a short stretch, but wouldn't be able to go that fast for very long

Its possible , but you will need to be streamlined, (lower head position arms tucked in) and you will need to generate a bit of power probably 300-400watts.

It would help if you were using slick tyres and a big gear and had the help of a tail wind. I would consider a short burst of anaerobic power like that to be within the scope of a reasonably proficient cyclist if all you want to do is hit that speed without sustaining it.

You can if you spin fast enough.

If you have the strength to do it yes its possible. In fact a lot greater speeds are obtainable. Check out the link.