Question 1159402
A man can do a job in 8 hours, he does 1/8 of the job in 1 hour

 a boy can do the same job in 12 hours, he does 1/12 of the job in 1 hour

 and a girl can do it in 16 hours She he does 1/16 of the job in 1 hour

 the man and the boy work together for one hour
So they do 1/8 +1/12 of the job = 5/24 of the job

Balance job = 1- (5/24 ) = 19/24 

 and then the boy and the girl finish the job?

boy and girl together do (1/12 + 1/16) of the job in 1 hour = 7/48 

7/48 of job 1 hour
19/24 of the job in t hours = (19/24)/(7/48)    = 5.4 hours

Total 5.4 + 1 = 6.4 hours