His math literally doesn't add up.
If you multi-task you need to ADD multiple multiplications, one for each task.
For example, if you perfectly divide all attention, time and effort in two tasks, you ideally get:
0.5 * 0.2 + 0.5 * 0.15 (given that the value of the first task is 0.2 and the value of the second task is 0.15)
One important point is likely, that there is overhead in multitasking, so the actual formula becomes 0.45 * 0.2 + 0.45 * 0.15 + 0.1 * 0
where 0.1 of the time is wasted in useless (value = 0) overhead (of switching tasks, getting into the zone and so on).
You may or may not enjoy spending time in this 'useless' overhead...