A team can change before a job is finished. The amount of work is still fixed, but the rate need not stay the same throughout. You will record each active interval, carry completed work forward, and use the new rate only on what remains. By the end you will handle a known joining or leaving time, a stated change in daily working hours, and one unknown joining time. You will distinguish additional time from total elapsed time, detect an impossible schedule, explain missing information, and stop when the job is complete even if another event was planned later. Use the previous lesson's meaning of one fixed job and constant independent rates. Unless a question deliberately withholds a condition, work is divisible, contributions do not overlap or obstruct one another, and there are no unstated breaks or setup delays. Rates stay unchanged within each stated interval. Worker-hours require equal, unchanged hourly productivity; headcount alone does not establish it.