x - wiek dziadka teraz
y - wiek babci teraz
[tex]\left \{ {{x+y=140} \atop {x=2(y-(x-y))}} \right. \\\left \{ {{x+y=140} \atop {x=2(y-x+y)} \right. \\\left \{ {{x+y=140} \atop {x=2(2y-x)}} \right. \\\left \{ {{x+y=140} \atop {x=4y-2x/+2x}} \right. \\\left \{ {{x+y=140} \atop {3x=4y/:3}} \right. \\\left \{ {{x+y=140 \atop {x=\frac43y}} \right. \\\left \{ {\frac43y+y=140} \atop {x=\frac43y}} \right.\\\left \{ {{\frac73y=140 /*3} \atop {x=\frac43y}} \right. \\\left \{ {{7y=420 /:7} \atop {x=\frac43y}} \right.\\[/tex]
[tex]\left \{ {{y=60} \atop {x=\frac43*60}} \right. \\\left \{ {{y=60} \atop {x=4*20}} \right. \\\left \{ {{y=60} \atop {x=80}} \right.[/tex]
Odp. Dziadek ma 80 lat a babcia 60.