Chain going out of the assembly I bought a 21-speed bike. The rear gear at 1st position puts the chain at 2nd toothed-wheel. Likewise 6th position puts at 7th wheel. When at 7th position, it goes out of the assembly. This problem started after a few rides. Can anybody help me out?
Your rear derailleur is out of adjustment, possibly because it was knocked against something. There are already many good guides to derailleur adjustment on the internet, so I'd rather link to one from one from Park Tool than write another one. If your derailleur has been knocked badly out of alignment, you might also need to realign the hanger.