1000 light bulbs with a new life of 120 days are installed in a factory. Their length of life is normally destribute with a standard deviation of 20 days.
a) How many will expire in less than 90 days
b)If it is decided to replace all the bulbs together what interval should be allowed between replacement if not more than 10% should expire before replacement.

Use the formula z=\frac{(x-{\mu})}{\sigma}

z=\frac{(90-120)}{20}.

Look it up in the z-table, then multiply that by 1000.

For part b, look up the z-score for .10, plug it in the formula and solve for x.