GMAT Shortcut (Answer Choice Testing)
Let downstream time = t
Downstream speed = 90/t
Since upstream took 0.5 hour longer:
Upstream time = t + 0.5
Upstream speed = 90/(t + 0.5)
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The problem gives:
Downstream speed = v + 3
Upstream speed = v - 3
Therefore the speed difference is always:
(v + 3) - (v - 3) = 6
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So test answer choices using:
90/t - 90/(t + 0.5) = 6
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Try A: t = 2.5
Downstream speed = 90/2.5 = 36
Upstream time = 2.5 + 0.5 = 3
Upstream speed = 90/3 = 30
Speed difference = 36 - 30 = 6
Works immediately.
Answer = A
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GMAT Takeaway
When:
1. The question asks for a time
2. Answer choices are given
3. The rates have a fixed difference
Consider plugging answer choices directly into:
Rate = Distance/Time
Instead of solving for variables.
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Recognition Trigger
If you see:
Distance fixed
Time difference given
Rate difference fixed
Think:
Test answer choices by converting times into rates and checking the rate difference.
Often much faster than algebra.