Answer
3/16
Work Step by Step
To find the missing diameter, we subtract the entire length by the two lengths of the rectangle not included with the circle. Doing this, we find:
$$ =\frac{15}{16} -\frac{3}{8}-\frac{3}{8} \\ =\frac{15}{16} -\frac{6}{16} - \frac{6}{16} \\ =\frac{3}{16} \\ $$
