Răspuns:
1 - 4 · [ (-5) ·(x-2)] = 21
1-4· ( -5x +10) = 21
1 +20x - 40 =21
20x -39 = 21
20x = 21 + 39 = 60
x= 60 :20 = 3
x ∈ [-∞, 3]