# standard deviation question

#### predentgal

The life of a battery is normally distributed with an avergae life of 50 hours and a standard deviation of 7 hours. What is the probablity that a randomly chosen battery will have a life between 57 and 64 hours?

a) 34%
b)68%
c)13.5%
d)47%
e)27%

I thought the answer was b because 57- 64 is 2 standard deviations above the mean which is 68%, but I guess I'm wrong.

Any suggestion to how its c?

#### Streetwolf

##### Ultra Senior Member
10+ Year Member
7+ Year Member
The life of a battery is normally distributed with an avergae life of 50 hours and a standard deviation of 7 hours. What is the probablity that a randomly chosen battery will have a life between 57 and 64 hours?

a) 34%
b)68%
c)13.5%
d)47%
e)27%

I thought the answer was b because 57- 64 is 2 standard deviations above the mean which is 68%, but I guess I'm wrong.

Any suggestion to how its c?
It is 68% between -1 and +1 standard dev of the mean.
It is 95% between -2 and +2 standard dev of the mean.
It is 99.7% (what I learned in stats, sometimes this number is different) between -3 and +3 standard dev of the mean.

You have a range between +1 and +2 standard dev of the mean.

If it is 68% between -1 and +1, then it is 34% between 0 and +1.
If it is 95% between -2 and +2, then it is 47.5% between 0 and +2.

If it is 34% between 0 and +1 and it is 47.5% between 0 and +2, then it is 13.5% between +1 and +2.

#### predentgal

Amazing explanation. Thanks so muchh I get it now. I see ur graduating from stonybrook next year. Thats where I want to go. Do u like it?

#### Streetwolf

##### Ultra Senior Member
10+ Year Member
7+ Year Member
Amazing explanation. Thanks so muchh I get it now. I see ur graduating from stonybrook next year. Thats where I want to go. Do u like it?
Aside from the typical problems any dental school may have, it's a great school to be at. You'll learn a lot (and probably more than you hoped to learn haha).

What year would you be entering?

#### predentgal

2011. If I get in that is. My gpa is good but I'm scared about the dat. Taking it in june..