This document  is discussing an idea to create a scalable housing solution for the world’s poor. This post is one in an occasional series on Vijay Govindarajan’s and Christian Sarkar’s idea to create a scalable housing solution for the world’s poor. Each post will examine the challenge from a different perspective, including design, technology, urban planning and more. Today, David A. Smith looks at the financial challenges of affordable housing. The cracking the challenge of slums is the world’s biggest problem of the next quarter-century, because the ecology of slums and the ecology of cities are linked. We cannot have a healthy global economy without healthy cities, and we cannot have healthy cities without tackling slums.