28. The average age of three boys is 15 years. If their ages are in the ratio 3 : 5 : 7, the age of the youngest boy is :

29. The average score of a cricketer for 10 matches is 43.9 runs. If the average for the first six matches is 53, the average for the last four matches is :

30. The average of 30 results is 20 and the average of other 20 results is 30. What is the average of all the results?