Cost of bananas is increased by Re.1 per dozen. One can get 2 dozen less for Rs.840.Find the original cost of one dozen bananas?

I'm having a hard time understanding your question, plus charging higher costs per dozen for more bananas doesn't really make sense... One would rather charge less for buying in larger bulk.

let original price = Rs. X...

bananas that can be bought = 840 / x...

increased price = Rs. (x + 1)...

bananas that can be bought with the increased price = 840 / (x +1)...

As,
840/x - 840/x+1=2

By solving this equation we get
original price = Rs. 20 (ans)...