Math Help

Archive for September, 2009

Question: Do you know any FREE 5th grade websites for math worksheets?

The student is going into 5th grade and needs helps with math skills, mostly long division and geometry? any websites?

Answer: I just always google ‘free worksheets’.

Math Worksheets – How to Calculate Percentages

 

If we divide [a,b] into n equal intervals, each of width: 

 

The first rectangle has area
The second rectangle has area
The ith rectangle has area
The nth rectangle has area

The area under f range [a,b] is therefore: 

Question: (Math)Statistics help- It’s urgent!10 points for correct answer!?

Tyre pressures on a certain type of car independently follow a normal distribution with mean 1.9 bars and standard deviation 0.15 bars.
1. Find the probability that all four tyres on a car of this type have pressures between 1.82 bars and 1.92 bars.

2. Safety regulations state that the pressures must be between 1.9-b bars and 1.9+b bars. It is known that 80% of tyres are within these safety limits. Find the safety limits.

The answers to these questions are:
1: (0.256)^4
2.limits between 1.71 and 2.09

Could you guys please help me solve this question particularly the second one? Please give me a guidance on how to arrive to the answer. Thank you!
And if you can’t help, please don’t bother answering!

Answer: 1.) z+= (1.92-1.9)/.15=.133 z-=(1.82-1.9)/.15=-.5333

USe your Z-score tables and you will find the areas to be .0517 for .13 and .2019 for .53 added together you have .2536 , they got .256 using a greater accuracy. So the probability that one tire is within these standards is .256. The probability that all 4 are is .256^4.

2.) So the area on the standard z-distribution from the mean to is .40, with .40 coming from the negative side. Using the table this gives a z-score of 1.285 and -1.285 for the negative side.

So the now we know that

z=(mean-value)/standard deviation
so
value=mean-z*standard deviation or 1.9-1.285*.15 = 1.71
and 1.9+1.285*.15 = 2.09

Probability & Statistics Tutor -Sample 3 -Standard Deviation