The example is wrong, it should state 20%, not 25%.
For example, a lance-sized force, with 4 units—one with C3M and a PV of 20, plus three units worth 15 PV apiece, all with C3S— are linked together. Per the rules for C3 equipment, the three C3S units are linked to the master (C3M) unit, adding 10 percent for the first link, plus 5 percent each for the other two units linked, for
a total of 20 percent. The first unit in the lance thus adds 4 points to its PV (20% x 20 PV = 4 PV) for a modified PV of 24, while each of the other three units adds 3 points (20% x 15 PV = 3) for a modified total of 18 PV each. The total PV for the lance is now 78 points.
Xotl: these C3 rules have been replaced in their entirety in the AS errata and book reprint.