notam:
g = numarul de gutui
c = numarul de cosuri
g = 13 × c + 16
g = 20 × (c - 2)
egalam cele doua relatii:
20 × (c - 2) = 13 × c + 16
20 × c - 40 = 13 × c + 16
20 × c - 13 × c = 16 + 40
7 × c = 56
c = 56 : 7
c = 8 (cosuri)
g = 20 × ( 8 - 2) = 20×6 = 120 (gutui)