Here is a set of data: 1300, 18, 25, 19, -7, 24. which observation is the outlier

An outlier is a point which falls more that 1.5 times the interquartile  range above the thrid quartile or below the first quartile. It is basically the point which is abnormally off the limit within which other points are within. Given our data points:
1300, 18,25,18,-7,24
compared to the rest of the points, 1300 is quite of the limit. Thus;
1300