If US and China, decide to buy carbon credits, from Spain and Ukraine to make up for their high emissions, then in how many years US, and China would be able to bring down its ratio of $$CO_2$$ emission (million ton) to per capita income to world standard benchmark of 0.75. (per capita income of the given countries remain same, 0.5 $$CO_2$$ emissions [million ton] is compensated by purchase of 1.25 units of carbon credit, and a country can buy carbon credit units in three lots of 15, 20 and 30 units in a single year.)