Which term is described by the equation S + T + M = I + G + X?

• A. Injections
• B. Final Goods
• C. GDP
• D. Equilibrium

Answer: (D)

In this equation, think of the economy as a circular flow with two kinds of flows: injections and withdrawals. Injections are investment, government spending, and exports (I, G, X). Withdrawals are private saving, taxes, and imports (S, T, M). When the total amount that leaks out of the economy (withdrawals) equals the total amount that is pumped in (injections), the level of national income stays steady—there’s no inherent pressure for it to rise or fall. That balance is what economists mean by equilibrium in this context: injections balance withdrawals.

So the equation shows that the economy is in equilibrium because the inflows and outflows of spending are equal. The other concepts don’t capture this balancing act: injections describe the sources feeding demand but don’t describe balance itself; GDP is a measure of total output but this identity is about the balance of flows, not a direct measure of output; final goods is not what the equation conveys.