Simple Interest Questions for SSC CHSL

Core Formula and Variables

Simple Interest is the extra money paid on a borrowed or invested amount, calculated only on the original principal — not on interest earned. Unlike compound interest, the base never changes. There are four variables: Principal (P), Rate (R), Time (T), and Simple Interest (SI). Know any three, find the fourth.

• Principal (P) = original amount deposited or borrowed
• Rate (R) = interest per year, expressed as a percentage
• Time (T) = duration in years
• SI = extra amount earned or paid
• Amount (A) = P + SI — this is what the person finally receives or repays
• Rate is always per annum (yearly) unless the question specifically says otherwise

Principal Doubles, Triples — Pattern Questions

SSC CHSL frequently asks: at what rate will a sum double in N years, or in how many years will a sum triple at R percent. These are shortcut-friendly questions. The key insight is: if a sum doubles, SI equals the principal (SI = P). If it triples, SI = 2P. Use this to set up the formula quickly.

• Sum doubles means A = 2P, so SI = P
• Sum triples means A = 3P, so SI = 2P
• Sum becomes N times means SI = (N-1) x P
• Time to double at R% = 100/R years
• Rate to double in T years = 100/T percent
• These shortcuts save 30-40 seconds per question

Two Installments or Two Parts of Principal

A common SSC CHSL pattern gives a total principal split into two parts, lent at different rates or for different times, and asks for either the split or the total interest. Set up two separate SI expressions and use the total interest as the equation to solve. This tests algebraic thinking but stays simple.

• Let one part be x, then the other part is (P - x)
• Write SI for each part separately
• Add both SIs and equate to total interest given
• Solve for x using basic algebra
• These questions may look scary but they collapse into a one-variable equation quickly
• Check: rates or times must be different, else the question makes no sense

SI vs CI — Difference Questions

SSC CHSL often asks the difference between CI and SI for 2 years or 3 years. For 2 years, the difference has a neat formula. These questions mix SI and CI concepts but only the difference formula is needed — you do not need to calculate CI from scratch. Memorise the two-year difference formula as a shortcut.

• For 2 years: CI - SI = P x (R/100)^2
• For 3 years: CI - SI = P x (R/100)^2 x (R/100 + 3)
• The 2-year formula is far more common in SSC CHSL
• This difference is always positive — CI is always more than SI
• If SI for 2 years is given, derive P x R / 100 first, then use in CI-SI formula

• Using months directly in the formula: Time must be in years. If T = 6 months, use T = 0.5 or T = 6/12 before applying the formula.
• Confusing Amount with Interest: The question may ask for Amount (P + SI) but you stop at SI. Always re-read the question last line.
• Forgetting to subtract 1 in the N-times formula: If sum triples, SI = 2P (not 3P). N-1 is the multiplier for SI, not N.
• Treating rate as decimal in formula: The formula already divides by 100, so enter R as a number like 8, not as 0.08.
• In two-part problems, setting up one equation but solving incorrectly by mixing up which rate belongs to which part: Always label clearly as Part 1 = x at R1, Part 2 = (Total - x) at R2.

• PRT formula mnemonic: Remember it as PARTY — P-A-R-T-Y where A = amount, the goal of every party (investment). SI = (P x R x T)/100.
• Doubles rule: Think of 100 as a wall. Rate x Time must hit 100 to double the money. R x T = 100 to double.
• N-times shortcut: Subtract 1 from N, then use T = (N-1) x 100 / R. Think: subtract before you multiply.
• CI - SI for 2 years: Think of it as the interest-on-interest for one year. CI earns extra interest on the first year SI. That extra = P x (R/100)^2.

• SSC CHSL typically has 1-2 SI questions per paper. They are placed in the easy-to-medium band — solve them in under 75 seconds each to save time for harder topics.
• The most common question types in recent papers: finding rate when sum doubles in N years, finding total amount after given years, and the CI-SI difference for 2 years.
• Questions rarely go beyond 3-variable problems. If a question looks complex, check whether values simplify nicely — they usually do in CHSL.
• Watch for unit traps: rate given as half-yearly or monthly. Convert everything to yearly before applying the standard formula.
• In the two-part split questions, if answer choices are round numbers like 1000, 1500, 2000, plug those back instead of solving algebra — saves time in exam hall.