Which is less: 0.25% or 0.06% ?
6 things is less than 25 things, so 0.06% is less than 0.25%.
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Which is stronger, 0.25% or 0.06% for medication?
Can't tell. You could be asking about concentration (0.25% would be stronger), dilution (0.06% would be stronger) or something else. The problem is we don't know what 0.25% and 0.06% refer to.