Statistics - Normal distribution

[repugno]

Greetings to all,

I have run into some extreme difficulty with this straightforward topic. My problem is that I can’t work out which tables to use. There is the normal table and then the percentage points table. When the question states that the probably under the curve is a percentage I would naturally use the percentage points table. However, this is where the problems arise. My answers are different from the textbooks answers, and then when I recalculate using the normal table my answers are right. Then my understanding changes, so only the normal table gets used even with the percentages. But yet again my answers are wrong, because I had to use the percentage table. Please, anyone, shine some light onto this dilemma. Thank you in advance.

 

[Mathechyst]

Can you supply a sample problem?

Doug

 

[repugno]

Thanks for the reply Doug. Here is one question which has the probabilities as percentages. For some reason I am not suppose to use the percentage points table. Apart from that the question is straightforward.

The duration of pregnancy of a certian breed of cow is normally distributed with mean \mu and standard deviation \sigma days. Only 2.5% of all pregnancies are shorter than 235 days and 15% are longer than 286 days.

Find the value of \mu and \sigma

Any help would be much appreciated, thank you.