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obecny wiek siostry : x
wiek Ani za 4 lata : (x + 8) + 4
wiek siostry za 4 lata : x + 4
2(x + 4) = x + 8 +4
2x + 8 = x + 12
2x - x = 12 - 8
x = 4
siostra Ani ma obecnie 4 lata a Ania ma 12 lat.