to install disc brake Hi friends I have a hercules roadeo a100 and I want to install a disc brake to it. It comes with a disc brake in front and v brake in rear. So can I install a disc brake to it and how much it costs.
First make sure you have disc brake mounting tabs on the seat stays. 



If you don't have those, then it's pretty much impossible. Technically still possible, but not without added costs that go well beyond what your bike is worth. 

Also, you need to make sure your rear hub has mounting holes for the disc.



If you don't have these, you will need to replace the hub or the wheel.  In your case it would probably be better to just replace the entire wheel.

Your bike is pretty low end.  If you are having braking issues, first make sure everything is adjusted properly.  Then perhaps consider getting a higher quality v-brake with good brake pads. This upgrade will be cheaper and will provide more than adequate braking power.