GMAT PREP I STANDARD DEVIATION

[pkw209]

Hey all,

Couldn't figure this one out. A brief explanation would be very much appreciated. Thanks!

Some water was removed from each of 6 tanks. If standard deviation of the volumes of water at the beginning was 10 gallons, what was the standard deviation of the volumes at the end?

a. For each tank, 30% of water at the beginning was removed
b. The average volume of water in the tanks at the end was 63 gallons

[papgust]

IMO A

An important point about SD's:
If a constant percentage is reduced/added from each term of the set, then the new SD of the set is also reduced/added by that constant percentage.

But if a constant (number) is removed/added from each term of the set, then the new SD of the set remains constant.

---

SD is 10 gallons before removing i.e. Old SD.

A. Says that 30% is removed from each tank.
Then SD is also reduced by 30%. which means 7 gallons. Sufficient.

B. this doesn't tell us any useful information to find the new SD. You are missing a key information here (Each volume of tank). Insufficient

[pkw209]

Great explanation! You are correct, answer is A!