Microwave Oven

[deepak.yadav]

In a certain microwave oven on high power, the time it takes a randomly chosen kernel of popcorn to pop is normally distributed with a mean of 140 seconds and a standard deviation of 25 seconds. What percentage of kernels will fail to pop if the popcorn is cooked for
(a) 2 minutes? (b) 3 minutes? (c) if you wanted 95% of the kernels to pop, what time would you allow? (d) If you wanted 99% to pop?

[factor-fiction]

Just use the formula for normal distribution or use the normal distribution tables..
From memory around 66% fall within one standard deviation, 95 % within 2 SDs and something like 99.6% within 3 SDs.
There must be something in your course notes/text books

[factor-fiction]

http://en.wikipedia.org/wiki/Normal_distributi on
These are correct values for distribution:

1 0.682689492137
2 0.954499736104
3 0.997300203937
4 0.999936657516
5 0.999999426697
6 0.999999998027

or looking at it the other way:

0.80 1.28155
0.90 1.64485
0.95 1.95996
0.98 2.32635
0.99 2.57583
0.995 2.80703
0.998 3.09023
0.999 3.29052


All you need is here with a bit of thought