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Bob is training for a fitness competition. In order to. . .

This topic has 1 expert reply and 0 member replies

Bob is training for a fitness competition. In order to. . .

Post Mon Nov 06, 2017 6:21 am
Elapsed Time: 00:00
  • Lap #[LAPCOUNT] ([LAPTIME])
    Bob is training for a fitness competition. In order to increase his maximum number of pull-ups, he follows the following routine: he begins with 25 pull-ups, rests for thirty seconds, and then does 24 pull-ups and rests, dropping one pull-up each time (25, 24, 23, etc.) until his final set of 11 pull-ups. How many total pull-ups does Bob do?

    (A) 55

    (B) 150

    (C) 270

    (D) 275

    (E) 325

    The OA is C.

    I can do it making the whole sum: 25+24+23+22+...+11, but, is there another way (faster or easier) to make it?

    Experts, can you give me a hand here?

    Need free GMAT or MBA advice from an expert? Register for Beat The GMAT now and post your question in these forums!

    GMAT/MBA Expert

    Post Mon Nov 06, 2017 9:11 am
    Great question - there is a faster way! When we have any group of terms, we can find the sum of all of those terms with:

    $$mean\ \cdot\ number\ of\ terms$$

    When we have a group of consecutive terms, we can find the mean of those terms with:

    $$\frac{first\ term+last\ term}{2}$$

    Combining those two equations, we can find the sum of a group of consecutive terms with:

    $$\frac{first\ term+last\ term}{2}\cdot\ number\ of\ terms$$

    Here, this gives:

    $$\frac{25\ +\ 11}{2}\ \cdot\ 15\ =\ 270$$

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