Răspuns :
[808-a]x2=418
808-a=428:2
808-a=209
808-209=a
a=599
verificare: [808-599]x2=418
209x2=418
418=418 (a)
808-a=428:2
808-a=209
808-209=a
a=599
verificare: [808-599]x2=418
209x2=418
418=418 (a)