EX 2.28 blood type again

For the blood-typing experiment of Example 2.27,

$$\begin{array}{rl}& P\left(\text{third type is }\mathrm{O}+\right)\\ =& P\left(\text{third is}\mid \text{first isn’t}\cap \text{second isn’t}\right)\\ & \cdot P\text{(second isn’t | first isn’t)}\cdot P\text{(first isn’t)}\\ =& \frac{1}{2}\cdot \frac{2}{3}\cdot \frac{3}{4}\\ =& \frac{1}{4}=.25\end{array}$$