(i) The tree diagram is structured as follows:

• First branch: Probability of using a cell phone (Ph) = 0.68, not using a cell phone (NPh) = 0.32.
• Second branch for Ph: Young (Y) = 0.7, Middle-aged (M) = 0.25, Old (O) = 0.05.
• Second branch for NPh: Young (Y) = 0.26, Middle-aged (M) = 0.10, Old (O) = 0.64.

(ii) To find the probability that a passenger aged 45 (middle-aged) uses a cell phone, use conditional probability:

\(P(\text{Ph} \mid \text{M}) = \frac{P(\text{Ph} \cap \text{M})}{P(\text{M})}\)

Where:

• \(P(\text{Ph} \cap \text{M}) = 0.68 \times 0.25 = 0.17\)
• \(P(\text{M}) = (0.68 \times 0.25) + (0.32 \times 0.10) = 0.17 + 0.032 = 0.202\)

Thus,

\(P(\text{Ph} \mid \text{M}) = \frac{0.17}{0.202} \approx 0.842\)