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1) 7+5=12
2)75-57=18
12-x+10×x=x+10(12-x)-18
12-x+10x=x+120-10x-18
12+9x=-9x+102
9x+9x=102-12
18x=90/:18
x=5 ; 12-x=12-5=7
Szukana liczba: 75