We have
11 = 8 + 3 = 2³ + 3
3 = 2 + 1 = 2¹ + 2⁰
so that
11 = 2³ + 2¹ + 2⁰
11 = 1•2³ + 0•2² + 1•2¹ + 1•2⁰
11₁₀ = 1011₂