If an atomic bomb equivalent to Hiroshima and Nagasaki was dropped in the middle of Manhattan, NY..what would the extent of the damage be?

Either one would flatten a fairly large area but would not totally destroy NYC by any means - you can try it out on this simulator: http://nuclearsecrecy.com/nukemap/

Not nice.

The combined yield of little boy and fat man was between 30 and 40 thousand tons of TNT and the result of that kind of explosion would be total devastation for an area of several square mile and partial devastation for anything up to 3 miles from the detonation point.

well you are in fabulous luck, as there's a website that was created to answer this very question for you

http://nuclearsecrecy.com/nukemap/

you can select the hiroshima or nagasaki bombs from the presets, or any of a bunch of other bomb designs

nothing

well them two bombs wernt the same as far as explosions but the way structures are built nowadays it wont be as bad as them two