Looks like the only way to do it is to plug in the numbers (e.g. Fox total undiscounted is $45, Pony total undiscounted is $36. Then if Pony is 9% off, Fox is 13% off, calculated the discounted totals, subtract them from the original, check if the diffferences add up to 9). B is the answer so it can be done in under 2 minutes if you can multiply and substract fast.

I suspect there is more eklegant solution using the derivatives but can't find it

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"Amicus Plato, sed magic amica veritas"