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jaykayes
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Bunuel
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Active GMAT Club Expert! Tag them with @ followed by their username for a faster response.
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Those who want to perform root of 1089 or any root in general faster. You need to apply the shrinking principle. Choose a range and slowly shrink as you go along.

root(1089) has to be between 30 and 40 and they give values from 900 to 1600.
Last digit is 9 thus it is either 33 or 37 only.
1089 is closer to 900 and 1600 thus we nee a value closer to 30 than closer to 40.
33 is closer to 40 and 37 is closer to 40. Or 37 would give a result that is more than half way towards 1600 or values above the mean of 900 and 1600.
Thus 33 is the root of 1089.

DSGB
A boat traveled upstream 90 miles at an average speed of (v-3) miles per hour and then traveled the same distance downstream at an average speed of (v+3) miles per hour. If the trip upstream took a half hour longer than the trip downstream, then how many hours did it take the boat to travel downstream?

A. 2.5
B. 2.4
C. 2.3
D. 2.2
E. 2.1
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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)
-----------------------------

The problem gives:
Downstream speed = v + 3
Upstream speed = v - 3
Therefore the speed difference is always:
(v + 3) - (v - 3) = 6
---------------------

So test answer choices using:
90/t - 90/(t + 0.5) = 6
-----------------------

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
----------

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.

---

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.
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Better to solve using options.
Option A is best to attack first.
Because 2.5+0.5 = 3 gives you an integer that is also divides 90 and thus high chances of being the answer.
As we see 90/3 = 30
90/2.5 = 36.
Both are separated by 6 which is also equal to 3-(-3) = 6.

Thus option A.
DSGB
A boat traveled upstream 90 miles at an average speed of (v-3) miles per hour and then traveled the same distance downstream at an average speed of (v+3) miles per hour. If the trip upstream took a half hour longer than the trip downstream, then how many hours did it take the boat to travel downstream?

A. 2.5
B. 2.4
C. 2.3
D. 2.2
E. 2.1
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