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\left \{ {{a+b=18} \atop {2a+2b=36}} \right.
\left \{ {{a=18-b} \atop {2(18-b)+2b=36}} \right.
\left \{ {{a=18-b} \atop {36-2b+2b=36}} \right.
\left \{ {{a=18-b} \atop {36=36}} \right.
rozwiązaniem są wszystkie pary liczb naturalnych, które spełniają warunek
a+b=18
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\left \{ {{\frac{1}{3}a+\frac{1}{4}b=9} \atop {2a-75\%b=9}} \right.
\left \{ {{a+\frac{3}{4}b=27} \atop {2a-0,75b=9}} \right.
\left \{ {{a+0,75b=27} \atop {2a-0,75b=9}} \right.
\left \{ {{0,75b=27-a} \atop {2a-(27-a)=9}} \right.
\left \{ {{0,75b=27-a} \atop {2a-27+a)=9}} \right.
\left \{ {{0,75b=27-a} \atop {3a=9+27}} \right.
\left \{ {{0,75b=27-a} \atop {3a=36}} \right.
\left \{ {{0,75b=27-a} \atop {a=12}} \right.
\left \{ {{0,75b=27-12} \atop {a=12}} \right.
\left \{ {{0,75b=15} \atop {a=12}} \right.
\left \{ {{b=20} \atop {a=12}} \right.
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\left \{ {{a+b=a-b+9} \atop {a=1,2+b}} \right.
\left \{ {{a+b-a+b=9} \atop {a=1,2+b}} \right.
\left \{ {{2b=9} \atop {a=1,2+b}} \right.
\left \{ {{b=4,5} \atop {a=1,2+4,5}} \right.
\left \{ {{b=4,5} \atop {a=5,7}} \right.
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