Difference of Cubes Ratio

CAT 2019 Slot 1 · Quantitative Ability · Medium · Number theory

This is a medium Quantitative Ability question from the CAT 2019 Slot 1 paper. It tests Number theory. The full answer key and a step-by-step explanation are below — try it yourself first, then reveal the solution.

The product of two positive numbers is 616. If the ratio of the difference of their cubes to the cube of their difference is 157:3157:3, then the sum of the two numbers is

  1. A.

    50

  2. B.

    85

  3. C.

    95

  4. D.

    58

Answer

A

Explanation

Let the numbers be aa and bb, with ab=616ab = 616. a3b3(ab)3=1573    (ab)(a2+ab+b2)(ab)3=(ab)2+3ab(ab)2=1+3ab(ab)2=1573\frac{a^3 - b^3}{(a - b)^3} = \frac{157}{3} \implies \frac{(a-b)(a^2+ab+b^2)}{(a-b)^3} = \frac{(a-b)^2 + 3ab}{(a-b)^2} = 1 + \frac{3ab}{(a-b)^2} = \frac{157}{3}. 3ab(ab)2=1543    3(616)(ab)2=1543    (ab)2=9×616154=9×4=36\frac{3ab}{(a-b)^2} = \frac{154}{3} \implies \frac{3(616)}{(a-b)^2} = \frac{154}{3} \implies (a-b)^2 = \frac{9 \times 616}{154} = 9 \times 4 = 36. ab=6a - b = 6. (a+b)2=(ab)2+4ab=36+4(616)=36+2464=2500    a+b=50(a + b)^2 = (a - b)^2 + 4ab = 36 + 4(616) = 36 + 2464 = 2500 \implies a + b = 50.

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