When p is equal to 12 , t is equal to 2 , and s is equal to 1 sixth , r is equal to 18 If r varies directly with p and inversely with the product of s and t , what is the constant of variation?

Direct variation with p and inverse variation with the product of s and t means r = kp/(st), where k is the constant of variation. Substituting the values gives 18 = 12k/((1/6)(2)) = 36k, so k = 18/36 = 1/2. The constant of variation is 1/2, choice D.

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