I got 20.77 from these numbers; https://en.wikipedia.org/wiki/Wii_sales
1st Full Fiscal Year of Wii Sales April 1st 2007 - March 31st 2008: 15.18 million (24.45-9.27)
2nd Full Fiscal Year of Wii Sales April 1st 2008 - March 31st 2009: 20.77 million (50.39-29.62)
You're doing it wrong, its 50.39-24.45 & 24.45-5.84
When the herd loses its way, the shepard must kill the bull that leads them astray.