Figure 5.12 shows a pinion meshing with a ring gear. The pinion has 30 teeth and 1.5 mm. The centre distance between the gears is 45 mm. Determine the pitch circle of both gears.

The problem describes two external gears in mesh (Figure 5.13).
The centre distance, $C = 700$ mm.
The speed ratio is $3:1$. Since gear A is larger than gear B in the figure, we can infer that the ratio of their pitch circle diameters is $D_A : D_B = 3 : 1$.
This means $D_A = 3 D_B$.

The formula for the centre distance between two external gears is:

Multiply both sides by 2 to simplify:

We have $D_A = 3 D_B$.

Substitute $D_A = 3 D_B$ into $1400 \text{ mm} = D_A + D_B$:

The pitch circle diameter of gear A is $\boxed{1050 \text{ mm}}$ and the pitch circle diameter of gear B is $\boxed{350 \text{ mm}}$.