Kapaln math #22

[Greensalad]

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22. If a diameter of a cylinder is increased by 20% and the height is decreased by 16 2/3 %, by what percent is the volume changed?
A. -10
B. 10
C. 20
D. 24
E. 83 1/2

Answer is C

I can do this problem but I would need more than a minute. What's the quickest way to do it?

 

[Greensalad]

Alright I can do it in 40 seconds if I plug in numbers, but I knew which numbers to plug in before I started doing the problem. I don't think plugging in numbers is a good idea.

 

[infernobutterfl]

22. If a diameter of a cylinder is increased by 20% and the height is decreased by 16 2/3 %, by what percent is the volume changed?
A. -10
B. 10
C. 20
D. 24
E. 83 1/2

Answer is C

I can do this problem but I would need more than a minute. What's the quickest way to do it?

We are supposed to do the math out by hand? (possible, but that is just a waste of time and effort.) if only the 24% was not there, then the answer would have been much easier. (one could guess-timate the answer)

 

[Streetwolf]

22. If a diameter of a cylinder is increased by 20% and the height is decreased by 16 2/3 %, by what percent is the volume changed?
A. -10
B. 10
C. 20
D. 24
E. 83 1/2

Answer is C

I can do this problem but I would need more than a minute. What's the quickest way to do it?

Volume = pi*r^2*h
New Volume = pi*(1.2r)^2*(0.833h)

We write 1.2 as (6/5) and 0.833 as (5/6).

So we have New Volume = pi*(6/5)^2*r^2*(5/6)*h
New Volume = (6/5)pi*r^2*h

The new volume is 1.2 * the old volume, so the new volume is increased by 20%.

C.

 

[Greensalad]

Volume = pi*r^2*h
New Volume = pi*(1.2r)^2*(0.833h)

We write 1.2 as (6/5) and 0.833 as (5/6).

So we have New Volume = pi*(6/5)^2*r^2*(5/6)*h
New Volume = (6/5)pi*r^2*h

The new volume is 1.2 * the old volume, so the new volume is increased by 20%.

C.

Thanks. Your QR's probably a 30.

 

[Streetwolf]

Thanks. Your QR's probably a 30.

28. Dunno if that was perfect or not 😛