Question

The time required to finish a test in normally distributed with a mean of 40 minutes and a standard deviation of 8 minutes. What is the probability that a student chosen at random will finish the test in more than 56 minutes?

84%

2%

34%

16%

Answer 1

Using Excel (=NORMDIST(56,40,8,TRUE)), I keep getting ~98%. This isn't an option, though, so...

Answer 2

i believe your excel function is a left tail measure

Answer 3

Not familiar with that term?

Answer 4

Oh, I see, the bell curve goes to the left

Answer 5

the left tail, the tail on th eleft .. the one that is towards the lesser than side then the greater than side ... the one that is <---- that way

left tailed

Answer 6

OK, gotch'ya

How does that affect my equation for the problem?

Answer 7

most likely 84% because the student wont rush and maby wants to get a good grade

Answer 8

the total area us 100%, you found left tail and all thats ramining is the right tail

Answer 9

... typos

Answer 10

Oh. Ohhhh.

I see. Thanks amistre! :D

Answer 11

http://openstudy.com/updates/55354220e4b0405b794d399b

Answer 12

lets dbl chk to make sure your 98% is good tho. i cant see why int aint but lets work it a little:

the distance from 40 to 56 is: 16

there are 16/8 = 2 standard deviations from the mean

by emipirical rule: about 95% is centered about the mean within +- 2 sds

that leaves 5% in some tails; left and right

5/2 = 2.5 should give us an apporixmate of the area of one tail

Answer 13

Yeah. I don't know why 2.5% isn't an option, but I selected 2% since it's the closest thing. I have a feeling they wanted me to find the normal distribution, round it, and then subtract from 100%, because that gives me an almost exact solution of 2%

Answer 14

But thanks for the explanation! This was really helpful. :D

Answer 15

2.5 is an estimate, an approximation. so it gets us close enough

Answer 16

if the options had been more exacting on us, we could have done something more precise ;)

Answer 17

Hello @HourglassMage. I'm sorry to disturb you but please if you see this message please get in touch with me ASAP. I really need your help. Please check your email.

Sincerely,
Kitty

Answer 18

since the question is done, i spose this doesnt count as an interruption :)