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Data InsightsTwo-Part Analysis

Free GMAT Two-Part Analysis Practice Question

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A transit authority inspects two bridges on fixed maintenance schedules. Both bridges were inspected in month 1. After that, the first bridge is inspected every 12 months, so in month 13, month 25, month 37, and so on, and the second bridge is inspected every 15 months, so in month 16, month 31, month 46, and so on. Consider every month after month 1. In one column, select the first such month in which the first bridge is inspected but the second bridge is not. In the other column, select the first such month in which both bridges are inspected in the same month. The two selections need not be the same row. Make only two selections, one in each column.

First month after month 1 in which only the first bridge is inspected: . First month after month 1 in which both bridges are inspected together: .

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Answer & Explanation

Correct answer

1: 13 · 2: 61

The two columns ask for two different things off the same pair of schedules, and neither answer tells you the other. The first column tracks only the 12-month bridge and asks when it is inspected on a month the 15-month bridge is not. The 12-month bridge is inspected in months 13, 25, 37, and so on. The 15-month bridge is inspected in months 16, 31, 46, and so on. Month 13 is a first-bridge month, and 13 is not in the second bridge's list, so the first column is 13.

Month 25 is also a first-bridge-only month, but it is the second such month, not the first, so 25 is the off-by-one trap for this column. Month 16 and month 31 belong to the second bridge, not the first, so picking either for this column reads the wrong schedule.

The second column asks when both bridges land in the same month. The first bridge repeats every 12 months and the second every 15 months, so a shared month after month 1 must be a multiple of the least common multiple of 12 and 15, which is 60, added to month 1. That gives month 61, then month 121, then month 181. The first shared month is 61, so the second column is 61. Month 121 is the second shared month, not the first, which is the off-by-one trap for this column.

Notice the first column never needs the value 60 and the second column never needs the single-bridge lists, so the two reads stand on their own.