[tex] \left \{ {{a+b=1400} \atop {b-a= \frac{a}{3} /(*3)}} \right.[/tex] ⇔ [tex] \left \{ {{a+b=1400} \atop {3b-3a=a}} \right.[/tex] ⇔ [tex]\left \{ {{a+b=1400} \atop {3b=4a}} \right. [/tex] ⇔ [tex] \left \{ {{a+b=1400} \atop {b= \frac{4a}{3} }} \right.[/tex] ⇔ [tex] \left \{ {{a+\frac{4a}{3}=1400/(*3) } \atop {b= \frac{4a}{3}} \right.[/tex] ⇔
[tex]{{\left \{ {{3a+4a=4200} \atop {b= \frac{4a}{3} }} \right.[/tex] ⇔ [tex] \left \{ {{7a=4200} \atop {b= \frac{4a}{3} }} \right.[/tex] ⇔ [tex] \left \{ {{a=600} \atop {b= \frac{4*600}{3} }} \right.[/tex] ⇔ [tex] \left \{ {{a=600} \atop { b=\frac{2400}{3} }} \right.[/tex] ⇔ [tex] \left \{ {{a=600} \atop {b=800}} \right. [/tex]
Proba:
a+b=1400
600+800=1400
1400=1400
b-a=a/3
800-600=600/3
200=200
