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


Sagot :

a+b=240 => - a - b = - 240; but, a+c=215 => -b + c = -240 + 215 = - 35; but, d-b+c=455 => d = 455 + 35 => d = 490.