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28 Mar 2017, 09:23
Kudos
Bookmarks
first approaching this problem I wrote down what the answer requires: 90/(v+3) or time spent going downstream
from the data let's make up an equation:
90/(v-3)-90/(v+3)=1/2
we can arrive to v^2=1089
v=33
Then be careful!
90/(33+3)=90/36=2,5
Answer is A
from the data let's make up an equation:
90/(v-3)-90/(v+3)=1/2
we can arrive to v^2=1089
v=33
Then be careful!
90/(33+3)=90/36=2,5
Answer is A
17 Jan 2018, 13:49
Kudos
Bookmarks
Hi All,
This is a layered story-problem and takes a lot of effort to solve using a traditional "math approach". Here's how you can solve it with a bit of logic and TESTing THE ANSWERS:
From the prompt, we can create 2 equations:
D = R x T
90 = (V-3)(T + 1/2)
90 = (V+3)(T)
We're asked for the value of T.
From the prompt, I find it interesting that the distance is a nice, round number (90)…. because when looking at the answer choices, most of them are NOT nice decimals. When multiplying two values together (as we do in BOTH equations), if you end up with a round number, chances are that either….
1) both numbers are round numbers
2) one of the numbers includess a nice fraction (e.g. 1/2) which can be multiplied and the end result will be a round number.
This gets me thinking that 2.5 is probably the answer, but I still have to prove it….I'm going to plug in THAT value for T and see what happens to the 2 equations….
90 = (V-3)(3)
90 = (V+3)(2.5)
30 = (V-3)
36 = (V+3)
33 = V
33 = V
Notice how both values of V are THE SAME? That means that we have the solution. V=33 and T=2.5
Final Answer:
GMAT assassins aren't born, they're made,
Rich
This is a layered story-problem and takes a lot of effort to solve using a traditional "math approach". Here's how you can solve it with a bit of logic and TESTing THE ANSWERS:
From the prompt, we can create 2 equations:
D = R x T
90 = (V-3)(T + 1/2)
90 = (V+3)(T)
We're asked for the value of T.
From the prompt, I find it interesting that the distance is a nice, round number (90)…. because when looking at the answer choices, most of them are NOT nice decimals. When multiplying two values together (as we do in BOTH equations), if you end up with a round number, chances are that either….
1) both numbers are round numbers
2) one of the numbers includess a nice fraction (e.g. 1/2) which can be multiplied and the end result will be a round number.
This gets me thinking that 2.5 is probably the answer, but I still have to prove it….I'm going to plug in THAT value for T and see what happens to the 2 equations….
90 = (V-3)(3)
90 = (V+3)(2.5)
30 = (V-3)
36 = (V+3)
33 = V
33 = V
Notice how both values of V are THE SAME? That means that we have the solution. V=33 and T=2.5
Final Answer:
GMAT assassins aren't born, they're made,
Rich
24 May 2020, 00:30
Kudos
Bookmarks
Quote:
Let's try this way
B = Speed of Boat
S = Speed of Stream
Upstream Speed = Net speed against the flow of current = B - S
Downstream Speed = Net speed with the flow of current = B + S
Time upstream = (1/2)Hour + Downstream Time
\(\frac{90}{v-3} = (\frac{1}{2}) + \frac{90}{v+3}\)
90/30 = 3
90/36 = 2.5
Diffference = 1/2
i.e. v-3 = 30 and v+3 = 36 i.e. v = 33
Downstream time = 90/36 = 2.5
Answer: Option A
