statistics
posted by stefanie .
2. The new Twinkle bulb has a standard deviation hours. A random sample of 77 light bulbs is selected from inventory. The sample mean was found to be 492 hours.
a. Find the margin of error E for a 90% confidence interval.
Round your answer to the nearest hundredths.

Margin of error = zvalue (sd/√n)
Find the zvalue corresponding to 90% confidence using a ztable.
sd = standard deviation
n = sample size
I hope this will help get you started. 
Please someone help me? The new twinkle bulb has a standard deviation 35 hours. A random sample of 50 light bulbs is selected from inventory. The sample mean was found to be x=500 hrs. Find the margin of error for 95%, construct a 95% confidence interval for the mean life of all Twinkle bulbs. I am desperate.