Consumer magazine: Because front-loading washers use less water than top-loading washers, ordinary powder detergent does not dissolve readily in front-loading washers.
Conclusion (So)
To get clothes really clean in a front-loading machine, you need to use a detergent formulated for front-loaders, not an ordinary powder detergent.
Evidence
Ordinary power detergents don't dissolve readily in front-loading machines.
Evaluate
It sounds like the author cares a lot about whether or not the detergent dissolves readily. What does readily mean? Easily, quickly.
The author apparently thinks that to get clothes really clean, it would be bad if your detergent weren't dissolving readily.
We might choose to look at the argument more formally. The conclusion involves conditional logic, because it uses the necessary indicator "need". Any conditional statement, X → Y, can be read "X requires Y". Our author is saying:
Getting clothes → Using special detergent really clean (requires) not, ordinary powder
The contrapositive creates this logic path:
Use ordinary Clothes not detergent -------------> really clean
The evidence does not define any "legal standard" of really clean / not really clean, but it does discuss a fact about using ordinary powder detergent: it doesn't dissolve readily.
So the evidence provides this part of the conclusion's logic path:
Use ordinary not dissolve detergent → readily
The missing assumption completes the logic path:
not dissolve clothes not readily → really clean
Goal
Look for something saying that it's bad when your detergent doesn't dissolve readily. It impedes your ability to get clothes really clean.
The argument also assumes a difference when it comes to the detergent formulated especially for front-loading washers. It assumes that this detergent does dissolve readily (and that it doesn't introduce any new factors that preclude clothes from getting really clean).