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# PS: solve for time to go downstream

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Intern
Joined: 02 May 2018
Posts: 2
PS: solve for time to go downstream  [#permalink]

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06 Oct 2018, 18:48
00:00

Difficulty:

(N/A)

Question Stats:

100% (01:41) correct 0% (00:00) wrong based on 2 sessions

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Boat travels 90 miles upstream at avg speed of (v-3). Goes back the same distance at avg speed (v+3). It takes half an hour more to go upstream than downstream. How many hours does it travel downstream?

Choices: A) 2.5 B) 2.4 C) 2.3 D) 2.2 1)2.1
Ans: A) 2.5

I set up these equations: t_u = t_d + 1/2 and t_u = 90/(v-3) and t_d = 90/(v+3) but once I started doing the math the numbers go too big and hard to divide manually, which made me think something was off.

Any help will be appreciated!
NUS School Moderator
Joined: 18 Jul 2018
Posts: 986
Location: India
Concentration: Finance, Marketing
WE: Engineering (Energy and Utilities)
PS: solve for time to go downstream  [#permalink]

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06 Oct 2018, 19:24
1
$$\frac{90}{(v-3)}$$ - $$\frac{90}{(v+3)}$$ = 0.5 (30 min to hours)

$$\frac{90(v+3)-90(v-3)}{v^2-9}$$ = 0.5

540 = $$v^2$$-9

$$v^2$$ = 1089

v = +33 or -33.

Velocity cannot be negative.

Hence v = 33.

Downstream time = $$\frac{90}{(33+3)}$$ = 90/36 = 2.5hrs.

Hope its clear.

Kudos if it helped

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Math Expert
Joined: 02 Sep 2009
Posts: 56304
Re: PS: solve for time to go downstream  [#permalink]

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07 Oct 2018, 03:09
niravchit wrote:
Boat travels 90 miles upstream at avg speed of (v-3). Goes back the same distance at avg speed (v+3). It takes half an hour more to go upstream than downstream. How many hours does it travel downstream?

Choices: A) 2.5 B) 2.4 C) 2.3 D) 2.2 1)2.1
Ans: A) 2.5

I set up these equations: t_u = t_d + 1/2 and t_u = 90/(v-3) and t_d = 90/(v+3) but once I started doing the math the numbers go too big and hard to divide manually, which made me think something was off.

Any help will be appreciated!

Discussed here: https://gmatclub.com/forum/a-boat-trave ... 00767.html

TOPIC IS LOCKED AND ARCHIVED.
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Re: PS: solve for time to go downstream   [#permalink] 07 Oct 2018, 03:09
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