20 salesmen can sell 100 boxes of ice cream working 30 hours a day, then how many boxes of ice cream can be sold when 60 salesmen work for 40 hours a day?
More salesmen means more boxes can be sold, so it is in direct proportion. The proportion is 20:60∷100:x
If more hours spend, then more boxes can be made, so it is in direct proportion. The proportion is 30:40∷100:x
We know that the product of extremes = the product of means.
20×30×x=60×100×40
x=(60×40×100)/(20×30)=400