Cara and shirley, working together, can clean the house in 66 hours. working alone, shirley takes three times as long as cara. how long does it take cara to clean the house alone

Assuming that they both work at the same pace together 66/2=33*3=99 to get the whole house done because she takes 3 times as long and has to pick up the slack when cara is gone i think at least it sounds logical in my mind but maybe somebody thinks different but final answer
t=99 hours