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.

x+y+z=24

7000x+14000y+28000z=504000

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