Answer:
4
Step-by-step explanation:
(A+B+C+D) / 4 = 10 .... average=sum/number of item
(A+B+C)/3 = 3D ... average of first 3 (A,B,C) is triple of forth (D)
A+B+C = 9D
(9D+D)/4=10
10D = 40
D = 4
