You know that the sum of a and b is 24, because the average is the sum of the numbers divided by the amount of numbers added together, and 12*2 is 24. Add 3 to 24 so you have 27, and then divide that by 3 because there are now 3 numbers in the set. The average of a, b, and c is 9.
Probably, think about what kind of ideas dewitt had and maybe think about what vigorous means properly before u give an answer. Hope that helped a little at least. Look up each word and concentrate on what they mean. Read more about him also.
I Think The answer is c or d I hope it helps My friend Message Me if I’m wrong and I’ll change My answer and fix it for you
