Concept:The mean is the sum of values divided by the number of values. Use the sums to isolate the missing 10th number.Explanation:Mean of 10 numbers is 56. So, sum of all 10 numbers =56×10=560. Mean of the first 9 numbers is 55. So, sum of the first 9 numbers =55×9=495. The 10th number is the difference between these two sums. 10th number=560−495=65. Since 65>56, this correctly increases the average from 55 to 56.Answer:The 10th number is 65. Correct option: B. 65.