During an experiment some water was removed from each of the 6 tanks .If the Std deviation of the volumes of the water at the beginning of the experiment was 10 gallons what was the standard deviation of the volumes of the water after the experiment ?

1) For each tank 30% of the volume of the water that was in the tank before the beginning of the experiment were removed during the experiment

2) The average ( mean ) volume of water in the at the end of the experiment was 63 gallons






(1) tells us that both the mean and the standard deviation of the set will decrease by 30%. Therefore, the new standard deviation will decrease to 7 gallons. SUFFICIENT.


Statement (2) tells us nothing about standard deviation, which measures SPREAD of numbers. If we achieved the 63 gallons by taking most of the water out of the tanks that were already lowest, then the standard deviation will be huge (because you'll have some tanks almost full and some almost empty). If we got there by taking most of the water out of the fullest tanks, then the standard deviation will be a lot smaller. INSUFFICIENT.


Ans : A


Note ----- Make sure you know that, when ALL numbers in a set are multiplied or divided by some number,** the mean and standard deviation are multiplied/divided by the same number


**This includes increasing or decreasing all the numbers in the set by some percentage (which can be accomplished by multiplication: e.g., 30% increase = multiplication by 1.3).