s + p + c = 28 lei
1s = 2p
1p = 2c ==< c = p : 2
inlocuim in prima relatie in functie de p
2p + p + p : 2 = 28
3p + p : 2 = 28 |×2
6p + p = 56
7p = 56
p = 56 : 7
p = 8 lei (un pix)
s = 2×8 = 16 lei (un stilou)
c = 8 : 2 = 4 lei (un creion)
verificare:
8 + 16 + 4 = 28 lei