Quantcast

Math Destroyer question

This forum made possible through the generous support of SDN members, donors, and sponsors. Thank you.

jsk26

Full Member
Joined
May 8, 2016
Messages
58
Reaction score
2
I just started the math destroyer practice test 1 in the 2016 edition. I do not understand the explanation for question 2. The mean=$320 the SD=$20 and we have to find the percentage of people who would pay $280 or less. How do we know that it is 3 SD's? and how can the percentage be determined?
 

DMikes

Full Member
2+ Year Member
Joined
Mar 14, 2016
Messages
521
Reaction score
582
The explanation in the back is pretty thorough....but I guess I'll try to explain it.

The mean is 320, you want less than 280. 280 lies right on 2 SD from the mean since SD = 20 (320 - 280 = 40, 40/20 = 2). You know that 95% of the population falls within 2 SD of the mean. In other words, you know that 95% of people are within 280-360. Therefore, there are 5% of ppl outside of 280-360 and the question explicitly asks for those who are less than 280.

2.5% will be less than 280 and 2.5% will be more than 360 since it is a normal distribution. Your answer is 2.5%
 

jz123

Full Member
2+ Year Member
Joined
Jan 22, 2016
Messages
170
Reaction score
29
1 step. google "statistics normal distribution curve"
2 step. to go from $320 to $280, it takes 2 SD ($40).
3 step. based on the distribution curve, $320-$300 is 34%, $300-$280 is 13.5%, then less than $280 is 2.5%
 
Top