Average Practice Test 4

Total age of 30 students = (30 × 23) years = 690 years
Total age of 10 students = (10 × 19) years = 190 years
Total age of 19 students = (19 × 25) years = 475 years
∴ Age of the 30th student = (190 + 475 - 690) years = 25 years