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.
The correct answer is D. Choose two points on the graph and count the rise and the run between them. Graph D is the only one that also has a slope of 3/4
.
The y-intercept is 43, which means that at 0 hours it was 43 degrees. The slope is -2 so it means that the temperature is decreasing at a rate of 2 degrees per hour.