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y-pisaki czarne
x+y=5/×(-2)
2x+1,5y=9
-2x-2y=-10
2x+1,5y=9
dodajemy oba do siebie
-0,5y=-1
y=2
x+y=5 stąd x+2=5
x=3
odp: kupiono 3 niebieskie i 2 czarne pisaki