I understand the problems that give various constraints and ask to maximize or minimize something, but not this type.
A chem manufacturer wants to rent a fleet of 24 tank cars with a combined carrying capacity of 504,000 gallons. Tank cars with three capacities are available: 7000 gallons, 14,000 gallons, 28,000 gallons. How many of each should be leased?
Set x, y and z as the three different capacity cars.
There are various answers. For example it can be 18 z's, or 17 z's and 2 y's, or 17z's and 4 x's.
The answer asks to express it as:
x=_t+_ y=_t+_ and z=t where _<=t<=_
that's something times t plus something, etc. where something <=t<=something