Explanation:

Average of 20 values = A = 18

Sum of all the 20 values= S = average * 20

=A*20 =18*20 =360

If 3 is substracted from each of the 20 values, then total value substracted = 3*20 = 60

Now new Total Sum = S1= S - 60

= 360–60 = 300

New Average = (New Sum) / 20 = 300/20 =15

Hence, option 3 is the correct answer.