In the figure attached, standard normal distribution table can be seen. Z is computed as follows:
In table can be seen that the area between 0 and 1.28 is 0.3997, or simply 0.4. The area until Z = 0 is 0.5; so, the total area until Z = 1.28 is 0.9. That means 90% of batteries would have 541.6 lifetime hours.