Need with this one question, also try to explain.

<ABC + <CBD = <ABD

substitute <ABD = 85 and <CBD = x

<ABC + x = 85

The bulbs should be replaced each 1436.9 hours.

Step-by-step explanation:

Problems of normally distributed samples are solved using the z-score formula.

In a set with mean and standard deviation , the zscore of a measure X is given by:

The Z-score measures how many standard deviations the measure is from the mean. After finding the Z-score, we look at the z-score table and find the p-value associated with this z-score. This p-value is the probability that the value of the measure is smaller than X, that is, the percentile of X. Subtracting 1 by the pvalue, we get the probability that the value of the measure is greater than X.

In this problem, we have that:

How often should the bulbs be replaced so that no more than 1% burn out between replacement periods?

This is the first percentile of hours. So it is X when Z has a pvalue of 0.01.

So it is X when Z = -2.33.

The bulbs should be replaced each 1436.9 hours.