teach the kids with real world examples.
suppose you worked on a crematory and received a costumer wanting to burn 6 million bodies in 5 years, you would have to calculate if you can provide him the service with your limited number of furnaces.
first we calculate the amount of time necessary, and since the demand is huge, we put people to work 24 hours/day, even though you may not have enough workers for it:
5 years · 365 days/year · 24 hours/day = 43800 hours
then you check how many bodies/hour you will need to burn:
6000000 bodies ÷ 43800 hours = 136.98 bodies/hour
at this moment you start to question if you are going to be able to serve this client, but to be sure, you check if you can distribute the load on 10 furnaces:
136.98 bodies/hour ÷ 10 furnaces = 13.69 (bodies/hour)/furnaces
then you verify the capacity of your furnaces, which for your sorrow, take around 3 hours to burn a single body.
in the end, you call the costumer back and let him know that his request is impossible.