data sufficiency questions that I dont understand

This topic has expert replies
Senior | Next Rank: 100 Posts
Posts: 71
Joined: Tue Mar 03, 2009 1:24 pm
If someone could please help me explain why the answers to these three questions are as they are.....


Over a certain time period, did the number of shares of stocks in Ruth's portfolio increase?

1) over the time period, ratio of number of shares to stocks to the total number of shares of stocks and bonds in ruth's portfolio increased

2) over the time period, the total number of shares of stocks and bonds in ruth's portfolio increased

answer: c



The lifetimes of all batteries produced by a company in a year have a distribution that is symettric about the mean m. If the distribution has a standard deviation of d, what percent of the distribution is greater than m + d?

1) 68% of distribution lies in interval from m-d to m+d inclusive
2) 16% of distribution is less than m-d

Answer: d

What is the value of the positive integer m
1) when m is divided by 6, the remainder is 3
2) when 15 is divided by m, the remainder is 6

answer b
Source: — Data Sufficiency |

Senior | Next Rank: 100 Posts
Posts: 88
Joined: Tue Mar 03, 2009 8:10 am
Thanked: 3 times

by quocbao » Wed Mar 04, 2009 8:03 pm
3:

I => m = 6p + 3 , let p = 1, m =9, p = 2, m = 15 so I is insufficient

II => 15 = km + 6 => 9 = km

Funny thing is that k and m can only be 3, so m is 3 => sufficient

Master | Next Rank: 500 Posts
Posts: 107
Joined: Wed Mar 04, 2009 4:39 am
Location: Vancouver
Thanked: 9 times
GMAT Score:750

Answer to question 2

by pakaskwa » Wed Mar 04, 2009 9:13 pm
For question 2, you need to know some basics about the standardized normal distribution is a bell shaped curve. Here's one of the links:
https://www.oswego.edu/~srp/stats/z.htm

Once you understand the curve, the question is easy:
1st condition told you the probability of distribution between (m-d) and (m+d) is 68%. Since the bell curve is symetrical, the probability of distribution that's less than (m-d) would be the SAME as the distribution more than (m+d). So
(1-68%)/2 = 16%

2nd condition told you the probability of distribution less than (m-d), and it's symetrical. So you'll get 16% right away.

Answer is D.

Master | Next Rank: 500 Posts
Posts: 107
Joined: Wed Mar 04, 2009 4:39 am
Location: Vancouver
Thanked: 9 times
GMAT Score:750

by pakaskwa » Wed Mar 04, 2009 9:17 pm
For question 1, I think there's a typo in the 1st condition:

"ratio of number of shares to stocks to the total number of shares of stocks and bonds in ruth's portfolio increased"

Should it be:
"ratio of number of shares OF stocks to the total number of shares of stocks and bonds in ruth's portfolio increased"

???