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
