A+B+C=......D=?

A+B+C=14……..(i)
A+B+D=17……..(ii)
A+C+D=20……..(iii)
B+C+D=21……..(iv)
(i) + (ii) + (iii) + (iv), we get
(A+B+C) + (A+B+D) + (A+C+D) + (B+C+D) = 14 + 17 + 20 + 21
thus,
3A + 3B + 3C + 3D = 75
3D = 72 - 3A - 3B - 3C
3D = 72 - 3(A + B + C)
3D = 72 - 3(14)........................(given A+B+C=14)
3D = 72 - 42
3D = 30
D = 10

3 + 4 + 7 = 14

3 + 4 + 10 = 17

3 + 7 + 10 = 20

4 + 7 + 10 = 21

A = 3

B = 4

C = 7

D = 10

d=10
c=7
b=4
a=3

10

因為b>a=1
c>b=3
d>c=3

所以d=10,then c=7,then b=4
10+7+4=21
b+c+d=21