Working alone joan can wash her car and clean the interior in 1 hour. of her sister it will only take 40 minutes. how long will it take if her sister does it alone

Working alone Joan finish the job in 60 minutes, which means every minute she does 1/60 of the whole job.
suppose the sister can finish the job alone in x minutes, every minute she does 1/x of the whole job.
together they do (1/60 +1/x) part of the job every minute.
for 40 minutes they finish the whole job: (1/60 +1/x)*40=1 (1 represents the whole job)
40/60 +40/x =60/60
40/x=20/60
x=120
so it will take the sister 2 hours to finish the job alone