This problem contains a constant rate of work expressed in units of wagons/hour. There is also a constant price per wagon. The objective here is to combine the rate of work, the time of production and the price to determine the total revenue. You can combine them in any order. For example, you might decide to determine the number of wagons x produced during the month first:

x  =  r t

where r is the rate of production and t is the production time in the month. Then the total revenue is given by

R  =  x p

Alternatively, you could figure out how much revenue f is being produced each hour

f  =  r p

then multiply the rate of revenue generation by the total time to get revenue

R  =  f t

Any approach will work well if you express each quantity with proper units and combine them so that the units cancel to get dollars. In reality, the key is to recognize that a price means dollars per wagon, and that the production time during the month can be expressed in hours.

R  =  r t p
$ = (wagon/hr) (hr) ($/wagon)
R  =  (150 wagon/hr) [(21 day)(8 hr/day)] ($13/wagon)  =  $(150 · 21 · 8 · 13)  =  $327,600