[tex]b_1 = 40 \\ q = \frac{1}{2} \\ b_4 = b_1 \times q ^{4 - 1} = b_1 \times {q}^{3} \\ b_4 = 40 \times ( \frac{1}{2} ) ^{3} = 40 \times \frac{1}{8} = 5 \\ b_4 = 8[/tex]