Homework Help: Upper and Lower Bounds, and the Percentage in Between

Hello I am struggling with the following problem:

If P(c<X<d) = .90; find C and D.

I know that the mean is 10 and the standard deviation is 2.
The instructions that I was given:


Step 1: First figure out the area to the left of the lower bound
Step 2: find the area to the left of the upper bound
Step 3: Use the invNorm function twice to find the upper and lower bounds.

I know how to use the invNorm function, but not sure how to find the area to the left of the upper and lower bounds without actually being given the upper and lower bounds? If that makes sense, I guess I just need to figure out how to approach this problem because I am not sure what they are asking….thank you very much for any assistance

mariposies said:

Hello I am struggling with the following problem:

If P(c<X<d) = .90; find C and D.

I know that the mean is 10 and the standard deviation is 2.
The instructions that I was given:


Step 1: First figure out the area to the left of the lower bound
Step 2: find the area to the left of the upper bound
Step 3: Use the invNorm function twice to find the upper and lower bounds.

I know how to use the invNorm function, but not sure how to find the area to the left of the upper and lower bounds without actually being given the upper and lower bounds? If that makes sense, I guess I just need to figure out how to approach this problem because I am not sure what they are asking….thank you very much for any assistance

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What are your thoughts? What have you done so far? Please show us your work even if you feel that it is wrong so we may try to help you. You might also read
http://www.freemathhelp.com/forum/threads/78006-Read-Before-Posting

Since you are talking about using the InvNorm function, I assume that you mean that for a normal distribution with a mean of 10 and a standard deviation of 2 one has P(c < X < d) = 0.90 and you want to know c and d. If that is all the information you were given there is no unique answer. For example, we might have c=\(\displaystyle -\infty\) and d~2*1.2816+10 = 12.5632 or we might also have c~-12.5632 and d=\(\displaystyle \infty\)