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S_A_V [24]
3 years ago
8

In a statistics​ class, the standard deviation of the heights of all students was 3.8 inches. The standard deviation of the heig

hts of males was 3.3 inches and the standard deviation of females was 3.1 inches. Why is the standard deviation of the entire class more than the standard deviation of the males and females considered​ separately?
Mathematics
1 answer:
goblinko [34]3 years ago
4 0

Answer:

The reason why standard deviation of the entire class is greater than standard deviation of males and females considered separately, is that mean values for males and females are different from each other.

Step-by-step explanation:

The concept of mean is well represented by the following formula

mean = \frac{(x1 + x2 + x3 + ..... + xn)}{N}, where x1, x2, xn are the observations and N is the number of observations (population).

Standard deviation represents the distance between each observation and the mean of the population (all observations). The formula for this parameter is:

Standard deviation =√[((x1 - x)² + (x2-x)² + ....+ (xn-x)²)/N-1], where x1, x2,..., xn are the observations and x is the mean value.

In this case you have that each height registered is an observation and the number of observations represents the N value. As you can see if the mean for males is different from that of females their standard deviation will be different too. Usually males have heigths greater than that of females (1.77 vs 1.64, in USA for example), and heights inside each group will be more similar than between groups. Then, when you mix all observation there will be an increase in standard deviation, because you are mixing very different heigths

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the ratio of sallys height is 7:8. if the sum of their heights is 90 inches. how tall are sally and jack?
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Answer:

Sally is 42 in. tall, and Jack is 48 in. tall.

Step-by-step explanation:

There are some words missing in the problem.

Perhaps the problem was meant to read:

"The ratio of Sally's height to Jack's height is 7:8. If the sum of their heights is 90 inches, how tall are Sally and Jack?"

Solution:

The ratio of the heights is 7:8.

Sally could measure 7 inches and Jack 8 inches, but then when you add the heights, 7 in. + 8 in. = 15 in. which is not 90 inches.

If you multiply both numbers in a ratio by the same number, the new numbers are still in the same ratio.

Multiply 7 and 8 by 2. You get 14 and 16. Maybe Sally is 14 in. tall and Jack is 16 in. tall. Add the heights, 14 in. + 16 in. = 30 in. This is also not 90 in.

We can keep on guessing what number to multiply by 7 and 8 by so their sum is 90, but we can use algebra to write an equation and not have to go through many guesses.

There is a number, x, that when you multiply 7 and 8 by x, the sum of the products is 90. We don't know what x is, but we can write an equation and solve for x.

The numbers in the ratio are 7x and 8x.

7x and 8x are still in the ratio 7:8.

Now we add the numbers 7x and 8x and set the sum equal to 90. Then we solve for x.

7x + 8x = 90

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x = 6

We get x = 6. Now we know we need to multiply both 7 and 8 by 6 to get the heights we need.

7 in. * 6 = 42 in.

8 in. * 6 = 48 in.

Check: 42 in. + 48 in. = 90 in.

Answer: Sally is 42 in. tall, and Jack is 48 in. tall.

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