Where the words come from
Low-income countries spending income on necessities, lack of saving meaning a lack of loanable funds for investment, and foreign aid and external investment being significant but not sufficient for capital accumulation — Principles of Economics 2e, OpenStax, licensed CC BY 4.0. https://openstax.org/books/principles-economics-2e/pages/32-2-improving-countries-standards-of-living. Changes: "Lack of saving means a lack of capital accumulation and a lack of loanable funds for investment in physical and human capital" narrowed to physical capital; the foreign aid sentence condensed; the dollar income figures dropped; the Harrod-Domar formula, the capital-output ratio, the sterling examples, the savings gap calculation and the way to fill the gap are our own wording, since 2e does not name the model; British spelling.
Diminishing marginal returns to capital deepening without new technology, and human capital and technology rebuilding Europe after the Second World War — Principles of Economics 2e, OpenStax, licensed CC BY 4.0. https://openstax.org/books/principles-economics-2e/pages/20-4-economic-convergence and https://openstax.org/books/principles-economics-2e/pages/20-3-components-of-economic-growth. Changes: "additional marginal amounts of capital deepening increase output by ever-smaller amounts" condensed into the diminishing returns card, with the rise in the capital-output ratio and the tractor example ours; the Europe example condensed from 20.3; the other two limitation cards are our own wording.
Questions, options, diagrams and feedback are our own. Figures credited to a source above are that source's; every other figure is invented to show the method.