The marks (out of 100) obtained by a group of students in a Mathematics test are 81, 72, 90, 90, 85, 86, 70, 93 and 71. Find the mean marks obtained by the group of students.

The mean of the age of three students Vijay, Rahul and Rakhi is 15 years. If their ages are in the ratio 4 : 5 : 6 respectively, then find their ages.

The mean of 25 observations is 27. If one observation is included, the mean still remains 27. Find the included observation.

The mean of 5 observations is 15. If the mean of first three observations is 14 and that of the last three is 17, find the third observation.