Since they can produce 1200 units in an hour (i.e. in 60 minutes), the time taken for each unit = 60/1200 = 0.05 minutes or in hours it takes 1/1200 (which is the same as 0.05/60).
So the throughput per hour = 80 / (0.05/60) which is the same as 80 x 60/0.05
Easier (and perhaps more obvious) is to say that each unit takes 1/1200 hours.
Therefore the throughput = 80 / (1/1200) = 80 x 1200 = 96,000