MHB What is the likelihood of a 14-year-old student being taller than 170cm?

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The discussion focuses on calculating the likelihood of a 14-year-old student being taller than 170 cm based on height distributions for boys and girls. For girls, the probability of being taller than 170 cm is approximately 0.066, while for boys, it is about 0.202. Given that 60% of the students are girls and 40% are boys, the overall probability that a randomly selected student is taller than 170 cm is around 0.12. Additionally, if a student is taller than 170 cm, the probability that the student is a girl is approximately 0.33. The calculations involve using z-scores and normal distribution properties to derive these probabilities.
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In a large school, the heights of all $14$yr old students are measured

The heights of the girls are normally distributed with a mean $155$cm and a standard deviation of $10$cm

The heights of the boys are normally distributed with a mean $160$cm and a standard deviation of $12$cm

(a) Find the probability that a girl is taller than $170$cm.

$\frac{155-170}{10}=1.5$

so with $\mu=0$ and $\sigma=1$ then $P(x>1.5) =0.0668072$

View attachment 1090

(b) Given that $10\%$ of the girls are shorter than $x$cm, find $x$

from z-table $10\%$ is about $.25$ so $.25=\frac{x-155}{10} x\approx157$

but i don't think this is the answer $143$ looks closer so ?

there is still (c), (d), and (e) but have to do later
 
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a) You have the correct z-score, but I would use:

$$z=\frac{x-\mu}{\sigma}=\frac{170-155}{10}=1.5$$

By my table, the area between 0 and 1.5 is 0.4332, hence:

$$P(X>170)=0.5-0.4332=0.0668$$

b) We want to find the z-score associated with an area of 0.4, which is about 1.28, and we attach a negative sign since this is to the left of the mean.

$$x=z\sigma+\mu=-1.28\cdot10+155=142.2$$
 
MarkFL said:
b) We want to find the z-score associated with an area of 0.4, which is about 1.28, and we attach a negative sign since this is to the left of the mean.

$$x=z\sigma+\mu=-1.28\cdot10+155=142.2$$

Where does the "area of 0.4" come from?

(c) Given that $90\%$ of the boys have heights between $q$ cm and $r$ cm

where $q$ and r are symmetrical about $160$ cm, and $q<r$

find the value of $q$ and of $r$.

well I did this half of $90\%$ is $45\%$ so $.45$ on Z table is about $1.66$ so
$160-1.66\cdot12\approx140.3=q$
and
$160+1.66\cdot10\approx179.7=r$

View attachment 1092

is this correct?
 
Last edited:
karush said:
Where does the "area of 0.4" come from?

We want 90% of the data to be greater than $x$. We know 50% is greater than $\mu$, and so that leaves 40% to be greater than $x$ and less than $\mu$.
 
karush said:
(c) Given that $90\%$ of the boys have heights between $q$ cm and $r$ cm

where $q$ and r are symmetrical about $160$ cm, and $q<r$

find the value of $q$ and of $r$.

well I did this half of $90\%$ is $45\%$ so $.45$ on Z table is about $1.66$ so
$160-1.66\cdot12\approx140.3=q$
and
$160+1.66\cdot10\approx179.7=r$

View attachment 1092

is this correct?

According to my table, the $z$-score is closer to 1.645 (using linear extrapolation).

Using numeric integration, I find it is closer to 1.64485.
 
MarkFL said:
According to my table, the $z$-score is closer to 1.645 (using linear extrapolation).

Using numeric integration, I find it is closer to 1.64485.

0.44950 from the wiki-z-table gave me 1.64 $160-1.64\cdot12\approx140.3=q$
$160+1.64\cdot12\approx179.7=r$
my prev post should of shown 1.64 not 1.66

still have (d) and (e) but have to come back to post it.
 
In the group of 14yr olds students $60$% are girls and $40$% are boys.

The probability that a girl is taller than $170$cm is $0.066$

The probability that a boy is taller than $170$cm is $0.202$

A fourteen-year-old student is selected at random

(d) Calculate the probability that the student is taller than $170$cm

this is probably not conventional method but if there are $600$ girls then $39$ of them are over $170$ cm and if there are $400$ boys then $81$ of them are over $170$ cm so that is
$\frac{120}{1000}\approx .12$

(e) Given that the student is taller than $170$ cm, what is the probability the student is a girl?
$\frac{39}{120}\approx .33$
 
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