Due to random variations in the operation of an automatic coffee machine, not every cup is filled with the same amount of coffee. Assume that the mean amount of coffee dispensed is 9 ounces and the standard deviation is 0.6 ounce. Use the 68-95-99.7 rule to complete the following.

b. What percent of the cups should have less than 8.4 ounces of coffee?

Step 1 ：Given that the mean amount of coffee dispensed is 9 ounces and the standard deviation is 0.6 ounce.

Step 2 ：According to the 68-95-99.7 rule, 68% of the data falls within one standard deviation of the mean, 95% falls within two standard deviations, and 99.7% falls within three standard deviations.

Step 3 ：One standard deviation below the mean is 9 - 0.6 = 8.4 ounces.

Step 4 ：68% of the cups should have between 8.4 and 9.6 ounces of coffee. Since the distribution is symmetric, half of this 68%, or 34%, should have less than 9 ounces of coffee.

Step 5 ：All the cups with less than 8.4 ounces of coffee should be the 34% that have less than 9 ounces, plus the 50% that have less than 8.4 ounces (since 8.4 ounces is one standard deviation below the mean, and half the data falls below the mean in a normal distribution).

Step 6 ：So, the percent of cups that should have less than 8.4 ounces of coffee is 34% + 50% = 84%.

Step 7 ：Final Answer: \(\boxed{84\%}\)