##25°C## 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##!!!



