# Destroyer QR Questions: HELP PLEASE

This forum made possible through the generous support of SDN members, donors, and sponsors. Thank you.
B

#### BalooTheBear

I searched the old posts and couldn't find anything for a couple of questions I had. If any out there gets this stuff better than me, please help me out. Thank you so much...

QR125: The ratios are equal between men and woman. Then the number of men decreases by 20% and the number of woman increases by 20%. What is the current ratio of men to woman?
I got 2/3 for this but it says the correct answer is 3/2. Do I have the ratio concept backwards or is this an error?

QR140: What is the equation of the line of symmetry of the equation:
x^2 = 4(y+2) - 16
Do we need to know parabola stuff, because I don't really know it that well.

QR152: The temperature for the first week of the month of January were recorded as follows: 5, 10, 15, 8, 13, 17, 9. What is the mean and standard deviation of the set of data?
Correct answer is 11 and 3.9.
My question is how and I supposed to determine standard deviation? I understand how to get mean, but am so lost with standard deviation.

I also have another question from my Kaplan Blue Book CD-Rom practice test:
If one car travels 24mph in one direction and then turns around and goes back to its original location at a speed of 48mph, what is the cars average speed?
The CD-rom did not give me the correct answer.

Thank you for your help. I'm going bald from stress. Only 3 weeks and 1 day till the real thing!!!

#### divine2

##### New Member
10+ Year Member
About the 1st Q..you're right! ratio is 2/3

About the Jan Temperature question..The st.dev, I keep getting is 4.2..
Stdev^2 = [(5-11)^2 + (10-11)^2 +(15-11)^2 +......]/7-1 =106/6
Stdev=Sqroot of (106/6)
=4.2ish

#### bigstix808

10+ Year Member
I also have another question from my Kaplan Blue Book CD-Rom practice test:
If one car travels 24mph in one direction and then turns around and goes back to its original location at a speed of 48mph, what is the cars average speed?
The CD-rom did not give me the correct answer.

Thank you for your help. I'm going bald from stress. Only 3 weeks and 1 day till the real thing!!!
if the car drove for 1 hr (t1), they would have driven 24 miles (d1)
if car turns around, it will cover that same 24 (d2) miles in half the time (t2)

average velocity = total distance/total time = d1+d2/t1+t2 = 48 miles/1.5 hrs = 32 mph

#### Streetwolf

##### Ultra Senior Member
10+ Year Member
7+ Year Member
QR140: What is the equation of the line of symmetry of the equation:
x^2 = 4(y+2) - 16
Do we need to know parabola stuff, because I don't really know it that well.

QR152: The temperature for the first week of the month of January were recorded as follows: 5, 10, 15, 8, 13, 17, 9. What is the mean and standard deviation of the set of data?
Correct answer is 11 and 3.9.
My question is how and I supposed to determine standard deviation? I understand how to get mean, but am so lost with standard deviation.
140. Rearranges to y = (1/4)x^2 + 2. So the vertex of this parabola is (0, 2). Thus the line is symmetry is x = 0. Graph it.

You need to look at the x value. If for instance it was (1/4)(x-2)^2 then the line of symmetry is around x = 2. In other words it is the x-coordinate of the vertex which is defined as the point (h,k) where the equation for your parabola is in the form y = a(x-h)^2 + k. MAKE SURE that y and x both have coefficients of 1 or else this will not work out!! Finally, a is the amplitude of the parabola. If a > 0 the parabola opens up (looks like a U). If a < 0 the parabola opens down (upside down U). The greater the magnitude (ie 100 compared to 10, or -100 compared to -10), the narrower it is.

152. 5, 10, 15, 8, 13, 17, 9. Answer 11 and 3.9.
You know average = 5+10+15+8+13+17+9/7 = 77/7 = 11. So that's not bad.

Standard deviation = sqrt [(The sum of all [xi - average]^2) / n-1], where xi = each individual value and n = the # of values.

So you want to do (5-11)^2 + (10-11)^2 + (15-11)^2 + ... + (9-11)^2 first which equals 36 + 1 + 16 + 9 + 4 + 36 + 4 = 106 and divide by (7-1) which comes to 106/6 = 17 and 2/3. Now take the square root of that. I know that gives you a different answer but that is how you are supposed to do it...

This problem obvs wants you to divide by just n. Okay fine. I'm not sure why and I have to run so someone else can add their 2 cents.

#### doc3232

##### Full Member
10+ Year Member
7+ Year Member
if the car drove for 1 hr (t1), they would have driven 24 miles (d1)
if car turns around, it will cover that same 24 (d2) miles in half the time (t2)

average velocity = total distance/total time = d1+d2/t1+t2 = 48 miles/1.5 hrs = 32 mph

So this is the correct way to do it, however, I can never remember it so I just always estimate and it will get you the correct answer.
For example, average of 20 and 30 mph would be about 23-24 mph.
You can always estimate, just take average and subtract a few mph.

#### klutzy1987

##### StudyingSucks Letsgo Mets
10+ Year Member
5+ Year Member
The way to calculate the average speed is to calculate the (total distance travelled)/(total time). In this case it would take 2 hours to travel 48 miles in one direction and one hour to return that 48 miles. Therefore your total distance is 96 miles and your total time is 3 hours. 96/3=32mph.

#### Saied

##### Full Member
10+ Year Member
i take statistics. look at the equation in google.

S= squareroot {(1/n-1) Sum of( XI-Xbar)^2}

n= is the number of samples u got
so its 7-1= 6
XI= u plug each number for example ur five to XI
Xbar= ur mean

hope it helps

#### Streetwolf

##### Ultra Senior Member
10+ Year Member
7+ Year Member
i take statistics. look at the equation in google.

S= squareroot {(1/n-1) Sum of( XI-Xbar)^2}

n= is the number of samples u got
so its 7-1= 6
XI= u plug each number for example ur five to XI
Xbar= ur mean

hope it helps
This problem wants the OP to divide by 'n' and not 'n-1'. You use 'n' when dealing with a population, which you certainly do not have in this problem. The 'n-1' is used for samples because it is a better estimator of the sample standard deviation than just 'n'.