Select the correct formula to calculate the value of Marginal Propensity to Save (MPS) : I. Change in Savings

Question 101 of 104

Q. Select the correct formula to calculate the value of Marginal Propensity to Save (MPS) : I. Change in Savings (ΔS) / Change in Consumption (ΔC) II. Change in Savings (ΔS) / Change in Income (ΔY) III. 1 - Marginal Propensity to Consume (MPC) IV. Change in Income (ΔY) / Change in Savings (ΔS) Options : (A) I, II, III and IV (B) II and III (C) Only III (D) Only II

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Concept understanding — Marginal Propensity to Consume

Marginal Propensity to Consume (MPC)

Start with everyday intuition

Think about what happens when you get some extra money — say, a ₹500 bonus from your part-time job, or a cash gift on your birthday. You don't save all of it, and you don't spend all of it either. Most people spend a part and save the rest. That part you spend — the fraction of the extra income that goes into consumption — is exactly what economists call the Marginal Propensity to Consume.

The word "marginal" here means "extra" or "additional." So MPC answers one simple question: Out of every extra rupee you earn, how much do you spend?

The precise meaning

Formally, MPC is the ratio of change in consumption expenditure to the change in income that brought it about.

MPC = \frac{\Delta C}{\Delta Y}

For example, if your income rises by ₹1,000 and your consumption rises by ₹750, your MPC is $750/1000 = 0.75$ (or 75%). This means you spend 75 paise of every extra rupee and save the remaining 25 paise.

The other side of the coin is the Marginal Propensity to Save (MPS) — the fraction of extra income that is saved. Since every extra rupee is either spent or saved:

MPC + MPS = 1

This is not a theory; it's an accounting identity. If MPC = 0.75, then MPS must be 0.25.

Why MPC matters

MPC is not just a number — it is the engine of the multiplier effect, one of the most powerful ideas in macroeconomics.

When someone spends money, that spending becomes someone else's income. That second person, in turn, spends a fraction (their MPC) of that income, which becomes a third person's income, and so on. A single initial injection of spending — say, government investment in a road — ripples through the economy, generating total income many times larger than the original spending.

The size of this ripple depends directly on MPC. The higher the MPC, the larger the multiplier.

Multiplier\ (k) = \frac{1}{1 - MPC} = \frac{1}{MPS}

If MPC = 0.8, the multiplier is $1/(1-0.8) = 5$ . An initial ₹100 crore investment can generate ₹500 crore of total income. If MPC = 0.5, the multiplier is only 2.

A word on the diagram …

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MPS measures the fraction of additional income that households save rather than consume. The correct formulas are II (ΔS/ΔY, the definition) and III (1 – MPC, from the income identity), so the answer is (B).

The Marginal Propensity to Save captures a simple behavioral question: when your income rises by one rupee, how much of that extra rupee do you tuck away as savings? It's the savings counterpart to the Marginal Propensity to Consume.

Start with the fundamental income identity. Every rupee of additional income must be either consumed or saved—there is no third bucket. Mathematically, for any change in income:

\Delta Y = \Delta C + \Delta S

Divide both sides by $\Delta Y$ :

\frac{\Delta Y}{\Delta Y} = \frac{\Delta C}{\Delta Y} + \frac{\Delta S}{\Delta Y}

1 = \text{MPC} + \text{MPS}

This tells us immediately that MPS = 1 – MPC (option III is correct). If households consume 0.75 of each additional rupee, they must save the remaining 0.25; the two propensities are complementary fractions of the same whole.

\text{MPS} = \frac{\Delta S}{\Delta Y} = 1 - \text{MPC}

Now examine the definition. MPS is the responsiveness of savings to income, so by definition it is the ratio of the change in savings to the change in income that caused it: ΔS/ΔY (option II is correct). This is the direct, textbook definition—just as MPC is ΔC/ΔY, MPS is ΔS/ΔY. …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

CA Foundation 2025Set may-20251 markMCQ
$Q. Under the Keynesian theory of determination of national income, the assumption is that the consumption increases with an increase in disposable income but the increase in consumption will be _______ the increase in disposable income. (A) Equal to (B) Opposite to (C) Greater than (D) Less than$
›Reveal solutionSolution

By Keynes's fundamental psychological law, a rise in income raises consumption by less than the rise in income.

Step 1 — Keynes's psychological law of consumption

Keynes held that as disposable income increases, consumption increases too, but not by the full amount of the increase — people save a part of the additional income.

Step 2 — Express with MPC

The marginal propensity to consume measures the share of extra income that is consumed:
$MPC = \frac{\Delta C}{\Delta Y}, \qquad 0 < MPC < 1$
Since $MPC < 1$ , the change in consumption $\Delta C$ is less than the change in disposable income $\Delta Y$ . …
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CA Foundation 2025Set may-20251 markMCQ
$Q. Which of the following is true in respect of relation of Marginal Propensity to Consume (MPC) and Marginal Propensity to Save (MPS) as per the Keynesian theory of determination of National Income ? (A) MPC = MPS (B) MPC + MPS = 1 (C) MPC + MPS = 0 (D) No relation exists between MPC and MPS$
›Reveal solutionSolution

Because extra income is either spent or saved, MPC + MPS = 1.

Step 1 — Split additional income

A change in disposable income ( $\Delta Y$ ) is divided between a change in consumption ( $\Delta C$ ) and a change in saving ( $\Delta S$ ):
$\Delta Y = \Delta C + \Delta S$
Step 2 — Divide through by $\Delta Y$
$\frac{\Delta C}{\Delta Y} + \frac{\Delta S}{\Delta Y} = 1 \;\Rightarrow\; MPC + MPS = 1$
So the two marginal propensities always sum to one, and MPS = 1 − MPC.

Why the other options are wrong: (A) MPC = MPS only in the special case where each equals 0.5; it is not a general rule. (C) MPC + MPS = 0 is impossible, since both are non-negative and their sum is 1. (D) A definite relation clearly exists, so 'no relation' is wrong. …
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