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> A manufacturer packages soap powder in containers of three different sizes. The amount of soap powder in a full large container could fill exactly 3 of the medium containers or exactly 5 of the small containers. If an equal number of small and large containers are to be filled with the amount of soap powder that would fill 90 medium containers, how many small containers will be filled?
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$90$ medium containers is equivalent to $30$ large containers.

If you divided $30$ into $25$ and five, the five large containers could fill $25$ small containers, so the answer is $25$ large containers, and $25$ small containers.

There's a longer algebraic way to solve this problem, and I'm seeing too much of a shortcut to do it. How to do it the algebraic way?

I don't know what to set as the "unknown" in the problem.
Do you set the number of containers of small($z$),medium($y$), and large($x$) to the three unknowns or do you express everything in one variable such as the number of large containers?

I also don't know how to express there must be an equal number of small and large containers in algebra.

$x=y?$

$x=5 \cdot y$?

$x+1/5x =?$