Simple Interest Questions for SSC CGL

Core Formula and Variables

Simple Interest grows in a straight line. Every year the same fixed interest is added on the original principal only — unlike compound interest which snowballs. The four variables are Principal (P), Rate (R% per annum), Time (T in years), and Interest (SI). If you know any three, you can find the fourth by simple algebra.

• SI = (P x R x T) / 100
• Amount A = P + SI = P(1 + RT/100)
• P = (SI x 100) / (R x T)
• R = (SI x 100) / (P x T)
• T = (SI x 100) / (P x R)
• Rate and Time are always in matching units — both per annum, or both per month

Sum Doubles, Triples, N-Times

A very common SSC CGL pattern asks: at what rate does a sum double in X years, or in how many years does it triple at Y%. The logic is simple — if a sum doubles, SI earned = P itself. If it triples, SI = 2P. Once you know SI as a multiple of P, plug into the formula and cancel P.

• Sum doubles => SI = P => R x T = 100
• Sum triples => SI = 2P => R x T = 200
• Sum becomes N times => SI = (N-1)P => R x T = (N-1) x 100
• If rate doubles and time halves, SI stays the same
• Time to double at 10% = 100/10 = 10 years — quick mental calculation

Effect of Changing P, R, or T

SSC CGL loves questions where two conditions are given simultaneously — same principal at two different rates or times — and you must find an unknown. Treat each condition as a separate SI equation and set them equal or subtract them. This gives you a system of two equations with two unknowns.

• Write condition 1: SI1 = (P x R1 x T1)/100
• Write condition 2: SI2 = (P x R2 x T2)/100
• Subtract or divide to eliminate P or find R
• If same P and T, ratio of SI = ratio of R
• If same P and R, ratio of SI = ratio of T

SI vs CI Connection

A popular hybrid question compares SI and CI on the same principal for 2 years. For 2 years: CI - SI = P(R/100)^2. This is a fixed formula to memorise. For 3 years the gap is bigger but SSC rarely tests beyond 2 years in this format. If you know SI for 2 years, CI = SI + P(R/100)^2.

• For 2 years: CI - SI = P x (R/100)^2
• SI for 2 years = 2 x (annual SI)
• CI is always greater than or equal to SI for same P, R, T
• If CI - SI is given along with R, you can find P directly

• Using A (Amount) instead of SI in the formula — always check whether the question gives total amount or only the interest earned
• Forgetting to convert months to years — if T is given as 6 months, use T = 0.5 in the formula
• In N-times problems, forgetting that SI = (N-1)P not NP — the principal itself is not interest
• Mixing up CI-SI formula direction — CI is always larger; if you get a negative answer you have flipped it
• Assuming rate is per month when it is stated per annum — always re-read the unit of rate

• PRT formula: Think PART — P-A-R-T, just replace A with nothing and divide by 100. Principal, Rate, Time over 100 gives SI.
• N-times rule memory hook: Extras times Hundred — (N-1) x 100 = R x T. The N-1 is the EXTRA beyond your original sum.
• Doubles at 10% in 10 years, at 5% in 20 years — inverse relationship: Rate x Years = 100 always for doubling.
• CI vs SI gap for 2 years: gap is P times Rate-fraction squared — imagine the rate fraction biting itself once extra.

• SSC CGL Tier 1 typically has 1-2 SI questions, occasionally a hybrid SI-CI question. Tier 2 Maths can have 2-3 questions with slightly more complex multi-part conditions.
• The most common formats in recent papers: sum doubles/triples, difference in SI under two rates, and finding principal when SI is a given fraction of principal.
• Time-saving trick: for straight formula questions, set up the equation mentally and avoid writing intermediate steps — these should take under 45 seconds each.
• Hybrid SI-CI questions for 2 years are a safe trap — examiners give Amount instead of SI to confuse you. Extract SI first before using any formula.
• If you see SI = (1/5) of P or similar fraction language, immediately convert: R x T = 20. Do not expand the full equation — use the shortcut R x T = fraction x 100.