Evaluating Mathematical Statements

XAT 2013 · Quantitative Ability & Data Interpretation · Hard · Number Theory

Please read the following sentences carefully: I — 103 and 7 are the only prime factors of 1000027 II — 6!6>7!7\sqrt[6]{6!} > \sqrt[7]{7!} III — If I travel one half of my journey at an average speed of x km/h, it will be impossible for me to attain an average speed of 2x km/h for the entire journey.

  1. A.

    All the statements are correct

  2. B.

    Only Statement II is correct

  3. C.

    Only Statement III is correct

  4. D.

    Both statements I and II are correct

  5. E.

    Both statements I and III are correct

Answer

C

Explanation

Statement I: 1000027=7×103×1387=7×103×19×731000027 = 7 \times 103 \times 1387 = 7 \times 103 \times 19 \times 73, so 19 is also a prime factor (False). Statement II: Raising to power 42 gives (6!)7>(7!)6  ⟹  (6!)7>76(6!)6  ⟹  6!>76=117649(6!)^7 > (7!)^6 \implies (6!)^7 > 7^6 (6!)^6 \implies 6! > 7^6 = 117649, which is False (720<117649720 < 117649). Statement III: Average speed V=2v1v2v1+v2=2xyx+yV = \frac{2 v_1 v_2}{v_1 + v_2} = \frac{2 x y}{x + y}. For V=2xV = 2x, 2xyx+y=2x  ⟹  y=x+y  ⟹  0=x\frac{2 x y}{x + y} = 2x \implies y = x + y \implies 0 = x, impossible. Thus Statement III is True.

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