How about using some logical reasoning rather than pure algebra.
E.g. X% and Y% mixture costs $7 and X% is increased by 50% => 1.5X% in the mixture increases the price by 1$.
A 50% increase in X results in a $1 increase in price. This implies the cost X% is $2 since 50% of $2 is $1. Hence the cost for Y% is $7 - $2 = $5.
Answer is C.
Any comments on fallacy of this approach...
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