We know that the standard deviation is 5 and the mean is 73. So, 68 is one standard deviation from the mean (left of the mean). While, 78 is one standard deviation away from the mean in the other direction (right of the mean).
Using a standard normal distribution (bell curve).
34% fall between 68 and 73
34% fall between 73 and 78
Therefore, 34% + 35% = 68%.
68% of the students scored between 68 and 78, which means most of the class did average/poorly on the test.
Side note: what helped me when I was in statistics was planting my mean and scores on the bell curve. I'll add a picture on how to do it.
