SAT Math Practice Exam

Answer C The greatest part of the job that can be done in one hour by the two fastest of the three machines i.e. by first two machines.
In 1 hr the first machine does ½ work and the second machine does 1/3 work.
So in hour both machines working together do ½ + 1/3 = 5/6 works

Key takeaway: In the questions based on time and work, take the total work to be done as one unit of work. Remember the basic concept that if A does a work in x hour , the in one hour A does 1/x of the work in 1 hour.