The answer to this question could be stated in two different ways. We could say that the labor productivity is 90 chairs per day. We could also, more accurately, say that the labor productivity is 15 chairs per worker per day.
Labor productivity is a measure of how many units of output are produced by a given number of workers in a given time. The time can be an hour, a day, a week, or any other amount of time that is convenient for the person doing the calculation. Productivity is derived by dividing the units of output by the units of input.
One way to find the productivity in this case is to use two steps. First, we divide the units of output (450 chairs) by the days worked (5). 450/5 = 90. So, our workers make 90 chairs per day. But this is less precise than knowing how many chairs each worker made. We have 6 workers, so we divide our 90 chairs per day by our 6 workers. 90/6 = 15. This tells us that our workers are making 15 chairs per worker per day.