MHB Need help on how to input invnorm and normalcdf using large numbers

[aprilryan]

Hi,

I'm a little unsure how to input large numbers into the TI-83 calculator using invNorm and normalcdf. Here's the question to the problem:

A study of VCR owners found that their annual household incomes are normally distributed with a mean \$41,182 and a standard deviation of \$19,990.

a. What is the percentage of households with incomes between \$30,000 and \$50,000?

Of course this cannot be inputted as normalcdf (30000, 50000, 41182, 19990) can it?

b. If an advertising campaign is to be targeted at those VCR owners whose incomes are in the top 90%, find the minimum income level for this target group.

This one also cannot be invNorm (.90, 41182,19990) can it?

Thanks

 

[Jameson]

Hi aprilryan,

a) I hadn't practiced this on the TI-83 before reading your question, but it seems this would be the correct syntax. What do you get for your answer? I can check it on another program.

b) Yes I believe this is the correct syntax. Again, I can check your answer if you wish.

 

[aprilryan]

The answer for a is .3824. The answer for b is 66800.21 (which I can sense is wrong). Yes, could you please check the answers for me? Thanks again.

 

[Jameson]

The answer for a is .3824. The answer for b is 66800.21 (which I can sense is wrong). Yes, could you please check the answers for me? Thanks again.

I got the same answer for both! :) Looks good to me. Why do you think (b) is wrong? It's good to check the reasonability of an answer, so questioning your result is smart but in this case it is in fact correct.

 

[aprilryan]

Well, I checked the answer key and for part b it says the answer is $15,564. Thanks again.

 

[Jameson]

Well, I checked the answer key and for part b it says the answer is $15,564. Thanks again.

Sorry I read the question incorrectly. It's asking you to find the top 90%, which is the same as the 10th percentile. When I read it I thought it was asking for the 90th percentile. So if you change your code to this it should work:

 invNorm (.10, 41182,19990)

 

[aprilryan]

I got it now! Thanks!