The mode of the data 4,6,6,8,9,6,9,9,10,9 is

Count the frequencies: 4 occurs once, 6 occurs three times, 8 once, 9 four times and 10 once. The most frequent value is 9. Trap: 6 is frequent but not the most frequent, 8 is the median-like middle value, and 7.5 is the arithmetic mean, which is not required to be a data value.