statistics, help

Can someone help me which way can I approach this problem. A hint of a formula or anything would be helpful...

Life of Light Bulbs A certain type of light bulb has an average
life of 500 hours, with a standard deviation of 100 hours. The
length of life of the bulb can be closely approximated by a normal
curve. An amusement park buys and installs 10,000 such
bulbs. Find the total number that can be expected to last for
each period of time.

The question is:

Find the shortest and longest lengths of life for the middle 80% of the bulbs.

I suggest you use the helpful tool at
http://psych.colorado.edu/~mcclella/java/normal/accurateNormal.html
Enter 500 for mean and 100 for std deviation. Then enter pairs of numbers for the upper and lower limits that are symmetrical bout 500. Hit
"Enter" to see that part of the distribution curve corresponding to those limits. I get 372 to 628 hours, corresponding to 80% of the bulbs, with 10% burning out earlier than 372 hours and 10% later than 628 hours.

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