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GMAT Question of the Day – 25/12/2020

QUESTION

Beth lives 40 blocks from her office. To get to work, she can walk, which takes 45 seconds per block, or she can take the subway, which takes z seconds per block. If she could wait for the subway for 20 minutes and arrive at her office at exactly the same time she would have arrived by walking, then z is

ANSWER SELECTION

(A) 10

(B) 13

(C) 14

(D) 15 (THIS IS CORRECT!)

(E) 20

ANSWER EXPLANATION

Because the time it takes to walk is equal to 20 minutes plus the time it takes to ride the subway, we need to begin by calculating the time required for Beth to walk to work. And, since the waiting time is in minutes, let’s calculate the walking time in minutes. In seconds, the walking time would be 40 blocks x 45 seconds per block. But because 45 seconds is 75% of one minute (60 seconds), the time in minutes it takes Beth to walk to work is (40 blocks)(0.75 minutes per block) = 30 minutes.

Since she could wait 20 minutes for the subway and still get to work at the same time, the subway must be able to make the trip in 30 – 20 = 10 minutes, or 10 minutes x 60 seconds per minute = 600 seconds.

Thus we can write this equation for the subway: (40 blocks)(z seconds per block) = 600 seconds, or 40z = 600. Dividing both sides by 40, we find z = 15, which is Choice (D). The subway takes 15 seconds per block.

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