Tìm x ∈ N, biết: 5.(12 – x) – 20 = 30

Ta có: 

5.(12 – x) – 20 = 30

5 (12 – x) = 30 + 20

5 (12 – x) = 50

12 – x = 50 : 5

12 – x = 10

x  = 12 – 10

x  = 2