The mode of a set of observations is that observation which occurs often.
There is a data set with 1000 observations. Out of these observations, the observation that occurs most frequently is 9, which occurs 13 times. After taking 20 more observations, it was observed that 13, which was not there in the observations before, occurred 19 out of 20 times and 9 occurred only 1 out of 20 times. What is the mode in these 1020 observations?