CDS Mathematics · Simple and Compound Interest

Simple Interest

Simple interest is charged on the original sum only, so it grows by the same amount every year: SI = PRT/100.

All Simple and Compound Interest notes CDS Mathematics strategy

Why this matters

Thirteen PYQs, one of them HARD. Everything is SI = PRT/100 read in a different direction: find the sum from extra interest at a higher rate, split a sum so each part earns equally, or settle two debts due at different times by bringing both to today.

Concept 1 of 1: SI = PRT/100

The interest each year is the same slice of the original sum, so after

T

years it is

T

slices. A change of rate by

1\%

for

T

years changes the interest by

\dfrac{PT}{100}

, whatever the rate was.

Definition

$\text{SI} = \dfrac{PRT}{100}$ ; amount $= P + \text{SI}$ .
A sum that becomes $k$ times in $T$ years earns $(k - 1)P$ in $T$ years, the same every year.
Rate higher by $\Delta R$ : extra interest $= \dfrac{P\,\Delta R\,T}{100}$ .
Parts earning equal interest over the same time: $P_1R_1 = P_2R_2 = \cdots$ .
Present worth of $A$ due in $t$ years: $\dfrac{A}{1 + \frac{Rt}{100}}$ .

Simple interest

\text{SI} = \dfrac{PRT}{100}

Worked example

2\%

more a year, a sum earns Rs.

90

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

SI = PRT/100
Simple interest
$\text{SI} = \dfrac{PRT}{100}$

Watch out for (1)

Triples means twice the sum in interest
→ SI = PRT/100

Test yourself on Simple and Compound Interest

15 past CDS questions from this chapter, timed at 18 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.

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