Question

Un pix si un stilou costa 5 lei , un stilou si un caiet costa 7 lei, iar un pix si un caiet costa 6 lei . Afla cat costa fiecare obiect.

Answer 1

$p+s=5 \\ s+c=7 \\ p+c=6 \\\\ \hbox{Se aduna toate cele 3 relatii} \\\\ 2p+2s+2c=6+7+5 \\\\ 2(p+s+c)=18 \ \ \ |:2 \\\\ p+s+c=9 \\ p+s=5 \ \ ===\ \textgreater \ c=9-5 \to \boxed{4 \ lei \ costa \ un \ caiet} \\\\ s+c=7 \ \ ===\ \textgreater \ p=9-7 \to \boxed{2 \ lei \ costa \ un \ pix} \\\\ p+c=6 \ \ ===\ \textgreater \ s=9-6 \to \boxed{3 \ lei \ costa \ un \ stilou}$

Answer 2

notam pix=P
           stilou=S
           caiet=C
P+S=5
S+C=7
P+C=6
calculam
S=5-P
C=6-P
inlocuim
5-P+6-P=7
11-2P=7
11-7=2P
2P=4
P=4:2
P=2
2+S=5
S=5-2
S=3
3+C=7
C=7-3
C=4