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by simba12123 » Tue Nov 11, 2008 7:30 am
When a cylindrical tank is filled with water at a rate of 22 cubic meters per hour, the level of water in the tank rises at a rate of 0.7 meters per hour. Which of the following best approximates the radius of the tank in meters?



a. (root 10)/2
b. root 10
c.4
d.5
e.10


qa is b
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by amitabhprasad » Tue Nov 11, 2008 7:35 am
Volume of the tank = pi *(r)^2*h
==> 22 = Pi *(r)^2 *0.7
this approx. to root(10)
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Tip

by iamcste » Tue Nov 11, 2008 8:14 am
Question has 22 and 0.7

clearly use Pi as 22/7 and run fast !

V= pi. r ^2.h

22=(22/7).r^2.(7/10)

r^2=10

r=sq rt 10 ( No need of approximating)
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Re: Tip

by stop@800 » Tue Nov 11, 2008 10:30 am
iamcste wrote:Question has 22 and 0.7

clearly use Pi as 22/7 and run fast !

V= pi. r ^2.h

22=(22/7).r^2.(7/10)

r^2=10

r=sq rt 10 ( No need of approximating)
The actual value of pie is not 22/7.
By using 22/7 we are actually approximating hence the word approximate is in Qn.
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hi

by simba12123 » Wed Nov 12, 2008 4:42 pm
Iamcste,

I like the approach but cant follow one step. hwo do we substitute pie for 22/7?
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by EricLien9122 » Thu Nov 13, 2008 2:55 pm
pie ~= 22/7

22/7 r^2 h=Area

22/7 r^2 0.7=22

22/7 r^2=220/7

r^2=(220/7) *(7/22)

r^2=10

r= square root of 10.


I think we need to know pie is about 22/7 in order to solve this question easily.

Please correct me if I made a mistake.
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by logitech » Thu Nov 13, 2008 3:04 pm
EricLien9122 wrote:I think we need to know pie is about 22/7 in order to solve this question easily.

22/7 will help you but lets say you only know that Pie is something like 3...

22=0.7(pie)RR
22=0.7(3)RR

22=2.1RR
10ish=RR

R=SQRT(10)

when the approximation is tested in GMAT the answer choices are designed with some room between them so these methods can be applied.

You will not see square root of 11 next to the sqaure root of 10.
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