Answer: You are funny :)
Step-by-step explanation: Visualise Average as concept of “give & take”.
Average of first 8 number = 21
Average of last 4 number = 18
If average of all 12 numbers is 18, then we have 8 x 3 = 24 extra from first eight numbers.
This extra is distributed over all 12 numbers increasing their average by 24/12 = 2.
=> Final average = 18 + 2 = 20
No equations no fuss.
Note : This method is for Aptitude Test takers and not for school homework.
:-)
