3 stilouri= 2 pixuri si 31 ruble
imlocuim stilourile cu pixuri
2p+31+5p= 80
7p+31= 80
7p= 80-31
7p=49
p= 49:7
p= 7 ruble un pix
3s+5p= 80
3s+ 5×7=80
3s+35= 80
3s= 80-35
3s= 45
s= 45:3
s= 15 ruble un stilou
verificare
3s= 2p+31
3s= 2×7+31
3s= 14+31
3s= 45
s= 45:3
s= 15 ruble