The 20 percent is the effect on the child's own adult income from growing up with more economically diverse friends. The 27,000 versus 47,000 comparison is a separate number: it's roughly the same size effect you'd see from that same child's own parents earning 47,000 instead of 27,000 while raising them, a different mechanism entirely, used only to give the 20 percent some intuitive scale. They're not the same number expressed two ways, so multiplying one by 1.2 to check the other won't line up.