# Thread: Quick beginner statistics question

1. ## Quick beginner statistics question

I'm taking AP Statistics right now, and my teacher isn't really all that great.

We just started normal distribution curves, and I have understood everything so far except for this question:

"While only 5% of babies have learned to walk by the age of 10 months, 75% are walking by the age of 13 months. If the age at which babies develop the ability to walk can be described by the Normal model, find the mean and standard deviation (the parameters)."

All of the equations we've learned so far require both the mean and the standard deviation, so I don't know how to solve this problem. I'm probably missing something obvious, but I don't know what it is.

Thanks

2. First of all

$\displaystyle z = \frac{x-\mu}{\sigma}$

and you have

$\displaystyle P(z<a)=0.05$ when $\displaystyle x = 10$ and $\displaystyle P(z<b)=0.75$ when $\displaystyle x = 13$

now solve for a and b

3. I'm not trying to be difficult, but I don't see the connection between what you wrote and the z score formula.

I don't get how I'm solving for a or b, I guess. What am I supposed to plug in?

4. Originally Posted by Centre
I'm not trying to be difficult, but I don't see the connection between what you wrote and the z score formula.

I don't get how I'm solving for a or b, I guess. What am I supposed to plug in?
Work backwards using a normal pdf table. This is a good one

Z table - Normal Distribution

I found that $\displaystyle P(z<a) = 0.05 \implies z = -1.645$