Air is cooled from room temperature 25 C to 100 K what is the temperature change in K?

##25C## is ##25+273=298 K##
so ##DeltaT=100-298=-198 K##
You can also see that ##100 K=100-273=-173 C##
and ##DeltaT=-173-25=-198 C##
The interesting thing is that the two scales being centigrades you have ##DeltaT## in ##K=DeltaT## in ##C##!!!