Question

a+b=240,c+d=500,d-b+c=455,a+c=215, d=?

Answer 1

a = 240-b
c = 500-d
240-b + 500-d = 215
740-b-d = 215
d-b + 500-d= 455
-b= 455-500
b = 45

740- 45 + 500 - d = 215
1195 - d = 215
-d = 215 - 1195
d = 980

Answer 2

a+b = 240, c+d = 500
d-b+c = 455
d+c = 455+b
a+c = 215
455+b = 500
b = 45
a = 195
c = 20
d = 480