could u tell me why? that datatype stuff is kinda confusion, or maybe a good link where it all gets explained?
Consider this part of your formula: RawADC*5L/1023L
It's using integer arithmetic, so any floating point component is dropped. It will only ever be 0,1,2,3,4 or 5. Consequently, if RawADC is 0, Resistance will be 0, but it'll also be 0 for all values of RawADC up to 195. Similarly, a range of values of RawADC will give you 2500.