In Example 5(b), change 320 to 360 and then find the resulting percentage of batteries.

Data from Example 5(b)

(b) We begin by finding the z-score of 320 days:

We wish to find the area to the right of z = −1.6, shown in Fig. 22.16. The area between z = 0 and z = −1.6 is 0.4452. Since the area to the right of z = 0 is 0.5, we add to get the total area: 0.4452 + 0.5 = 0.9452. This means that 94.52% of the batteries will last longer than 320 days.

Z = 320 - 400 50 -1.6