Question 1006157
(a + b)/2 = 9
(b + c)/2 = 6
(C + D)/2 = 15


from this you get:


a + b = 18
b + c = 12
c + d = 30


solve for b in a + b = 18 to get b = 18 - a


solve for c in b + c = 12 to get c = 12 - b


start with c + d = 30


replace c with 12 - b to get:


12 - b + d = 30


replace b with 18 - a to get:


12 - (18 - a) + d = 30


simplify to get:


12 - 18 + a + d = 30


combine like terms to get -6 + a + d = 30


add 6 to both sides of this equation to get:


a + d = 36


divide both sides of this equation by 2 and you get:


(a + d)/2 = 18


the average of a + d is equal to 18.


i believe that's the answer you are looking for.