Ex10 va rog dau test am nevoie de ajutor

a - elevi are au participat
b - elevi care nu au participat
a+b = 27 -> a = 27- b
a + 9 = 2 × b
27 - b + 9 = 2× b
36 = 3× b
12 = b = elevi nu au participat la concursul de sah
27 -12 = 15 elevi au participat la concursul de sah