Can marines kill spider mines by themselves? I need to know for a map. They have 40 hp. Like can 6 marines take on 10 spider mines, etc.

yes, as long as the marines have enough health/do enough damage.

I figured it would take 7 hits to kill a mine, so is it unlikely to work? (6 Marines vs.10 mines?)

Yes, as long as the mines aren't too bunched together.

I know from experience, because one of my friends hates the fact i completely cover the map with mines on Dropship Wars =P

He did it with five.

With proper micro (assuming the Marines are not computer-controlled and no stats have been modified) and decent placement, yes, this is possible.

If the Mine has enough HP to survive an attack from each Marine (40, as you say), I doubt it. Unless there are some sort of obstacles of buildings or terrain blocking the Mines, the Marines won't get off second shots to finish off the mine.

Range upgrade also helps, as well as detection

The mines have 40hp.

I might use ghosts.