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Two equal ratios satisfy ad = bc, the cross product. Solving that for the unknown gives d = bc/a, and the two ratios should then agree exactly.
Cross multiplication
a/b = c/d ⇒ ad = bc ⇒ d = bc / a
d = 4 × 9 / 3 = 12, and both ratios equal 0.75.
Multiplying both sides of the equation by bd clears the denominators, leaving ad = bc.