The original math challenge had to do with the total number ordered, the number delivered (and by implication the number still to be delivered), and the number with completed testing. Those numbers are all consistent with each other. IOW, the math for those numbers has no challenges.

The question of how long it will take to complete certification testing of all units ordered is a different one, and can't be accurately answered based on the time needed for the first 3. The first ones almost certainly revealed issues that will need to be patched in the second group, and which will need to be worked into the production process for the rest. If all the problems have been found, the 17 remaining to be delivered should proceed through certification more quickly than the first 2 batches. "Should" of course is the operative term there, and permits some variation.

Certification testing is required for all rail transit vehicles. It's why, for instance, Sacramento ordered its new low-floor LRVs for delivery many months to as much as a year before the system is ready for them (station platforms modified to work with them). Call it Dealer Prep, but at the user's site.