Assuming 129 is given in base 10, notice that
129 = 128 + 1 = 2⁷ + 1
More clearly, we have
129 = 1•2⁷ + 0•2⁶ + 0•2⁵ + 0•2⁴ + 0•2³ + 0•2² + 0•2¹ + 1•2⁰
so that
129 = 1000 0001₂
(I put a space between each block of 4 bits for ease of reading)
