Question

A town has a speed limit of 30mph through town. A radar detector measures the speed of 150 cars during the course of several hours. The average speed of all of these cars is 33.5mph with a standard deviation of 4.5 mph. What percentage of cars are speeding?

Answer 1

well, 30 is our test value

Answer 2

Yeah

Answer 3

(30 - 33.5)/(4.5/sqrt(150))

Answer 4

Wait, why do you need the square root of 150??

Answer 5

it might not need to be used in this case .... but when finding the z value of a sample size they give a formula for it to modify the standard deviation

Answer 6

if we use it we get -9.xx without it we get something more reasonable ... -2.12

Answer 7

Okay :o

Answer 8

so, assuming the -2.12 us good, how would you proceed?

Answer 9

Use a z chart to decide what percentage that is? I don't really know.

Answer 10

its the same process you used on that last one i was on .... yes, use the zchart

Answer 11

Okay, so 98.30 :o

Answer 12

yep

Answer 13

and then we multiply 150 by .983?

Answer 14

correct

Answer 15

147.45... Round up to 148?
And that would be how many were not speeding? Or were speeding?

Answer 16

that wold be the number of people going over 30mph ....

Answer 17

number of cars ... but yeah

Answer 18

So those who ARE speeding

Answer 19

yes, if they are not spose to go over 30 .... then those going over 30 are speeding

Answer 20

Ohhh, okay. This makes a tad more sense now. Thank you

Answer 21

good luck :)

Answer 22

Then to find the percent of those speeding, I would just do 148/150?
Or to be more approximate, do 147.85/150?