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GMAT Problem of the Week. Issue#23

Bob drove from town X to town Y at a constant speed, and then drove back to X along the same route at a different constant speed. Did Bob travel from X to Y at a speed greater than doudounecanadaparis 50 km per hour?

(1) Bob’s average speed for the entire round trip, canada goose pas cher excluding the time spent at town Y, was 100 km per hour.

(2) It took Bob 15 more minutes to drive from X to Y than to make the return trip.

(A) Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.
(B) Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.
(C) Both statements TOGETHER are sufficient, but NEITHER one ALONE is sufficient.
(D) EACH statement ALONE is sufficient.
(E) Statements (1) and (2) TOGETHER are NOT sufficient.

4 thoughts on “GMAT Problem of the Week. Issue#23

  1. The answer is A.

    Let the speed on Bob’s way from X to Y is V1, and his speed back form Y to X is V2. We are asked to determine whether V1 > 50 (km/h)

    (1) the formula for the average speed for the round trip is
    V(a) = (2V1V2)/(V1+V2)*.

    *(If you don’t know it or forget on the GMAT, you can get this formula by substituting D1=D2=d, t1=D1/V1=d/V1, and t2=D2/V2=d/V2 into the general formula V(a) = (D1+D2)/(t1+t2) and factoring).

    Then, substituting V(a) = 100, and assuming that V1 we have
    100 = (2V1V2)/(V1+V2)
    50 = (V1V2)/(V1+V2)
    V1V2 = 50(V1+V2)
    V1V2 = 50V1 + 50V2
    V1 = 50(V1/V2) + 50
    Since V1/V2 > 0, (50(V1/V2) + 50) > 50, and V1 > 50
    Sufficient

    (2) Since we know nothing from this statement about Bob’s speed during both parts if his trip, the statement (2) is insufficient to answer the question.

    Thus, the answer is (A).

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