Answer:
Step-by-step explanation:
Total sum of age of 12 students = 12*18 = 216;
Total sum of age of 8 students = 8*23 = 184;
Total sum of age of 20 students = 216+184 = 400;
Mean age of all the students = total age / number of students;
= 400/20 = 20 year
Step-by-step explanation: