cylinder formula!

[marmar29]

hi i just wanna chech tht i m on the right track
if he s telling an aquarium on the shape of cylinder with base diameter 25 cm & ht 35 cm
wht s the needed volume to fill it ?
a- 17171.875cm3
b 17171.875m3
c- 171.71875m3
d 1.7171875c m3
e - 1717187.5cm3

i ve the answer 17,187
wht abt u ?
have a good day

[arashyazdiha]

it is (pi)*r^2*h
so pi*(25/2)*(25/2)*35 and the unit is in cm^3
~ 3*(625*35)/4 ~ 156*3*35 ~450*35 ~15750( extreme roughly way of calculation when the choices differ in the number of digits)
so it is about 16000 cm^3 so A is the best asnwer
but the exact answer is 17171.875 which is A

Bests
Arash

[marmar29]

thanks Aresh but =i ddnt understand after u got 16000 why u choose A???
PLS CLARIFY
THANKS ALOOT

[Abhishek009]

hi i just wanna chech tht i m on the right track
if he s telling an aquarium on the shape of cylinder with base diameter 25 cm & ht 35 cm
wht s the needed volume to fill it ?
a- 17171.875cm3
b 17171.875m3
c- 171.71875m3
d 1.7171875c m3
e - 1717187.5cm3

i ve the answer 17,187
wht abt u ?
have a good day

D = 25

So R = D/2 =12.5

H = 35


Now volume of the cylinder is cross sectional area of a circle * height

=> 22/7*12.50*12.5 * 35

=> 17187.5 cm^3

[arashyazdiha]

Hi again
What I mean is that in these kinds of questions after seeing the answer choices and differences between them you should understand that maybe the better way of solving such a problem is not solving the problem with exact details with lots of hard multiplications as this job takes you lots of time. And one of the things that I have encountered in GMAT is your ability to overcome these kinds of problems. the Designer of the test somehow is not trying to ask you how exact you can solve this problem but how fast and best you can find a quick solution to it.
If you calculate the given formula with putting the estimated numbers then you can get a good perception about the best answer.
you don't need to multiply the r^2*h by exact pi which is 3.14 you can just use 3. you don't need to know that what exactly is 12.5*12.5*35 , just calculate it as a close estimate of each item, I will say:
(25/2)*(25/2)= 625/4 which is estimate of (600/4) = 150 plus 25/4 which is almost 6 so will be 156.
multiplying these whole things with 35 will be 156*35*3, again I will say 35*3 is around 100 so multiplied by 156 will be 15600
this is a final number that I have roughly calculated and it is in cm^3
Looking at the answer choices the only one that is close in magnitude and number of digits to what I have calculated is 17171.875 cm^3 and the difference is perceivable, because I reduecd 3.14 to 3 and ....(some more slight simplifications)
I hope I was able to explain it better.
Bests
Arash

[arashyazdiha]

The explanations might be long-winded, but be sure if you do it in your mind with these estimations it will save you lots of time on a real test.