By looking at the income distribution and realizing that a vast majority of the worlds income is concentrated in the hands of a breathtakingly small group of people and is continuing to be concentrated even more by the advent of even more integrated automated production and services.
All the while this small group continues to lobby for lower taxes worldwide and engages in tax-avoidance schemes that drive local manufacturers and service providers out of business. Heck, this group has so much money, they don't even know what to do with it anymore! Profits have skyrocketed in the last 30 years, but these increases have gone exclusively to the richest people of the world.
Productivity per person has more than quadrupled since the 1950. Yet the wages have stagnant since the 1970. Where did all the money go? I bet you can answer that question yourself.
Now, where do you specifically go wrong with the math? By not having a high enough tax on those that mooch away all the money. They could afford it. They just don't want to. And the system they built for themselves allows them to get away with it.
https://en.wikipedia.org/wiki/Distribution_of_wealth
https://en.wikipedia.org/wiki/Workforce_productivity