Mathematics
Mathematics in the RRB Lab Technician CBT tests numerical ability under time pressure: number system, simplification, HCF–LCM, percentage, ratio and proportion, average, interest, time–speed–distance, time and work, mensuration and data interpretation. Questions are short but calculation-heavy, so getting the right answer in one or two steps matters more than a long derivation.
Core rules
- BODMAS; for two numbers, LCM × HCF = product of the numbers.
- Successive percentage change: an a% increase followed by a b% decrease gives a net change of a − b − ab/100.
- Alligation: ratio of quantities = (mean − cheaper) : (dearer − mean).
- Simple interest SI = PRT/100; compound interest CI = P[(1 + R/100)^n − 1]; for two years, CI − SI = P(R/100)².
- Speed: multiply km/h by 5/18 to get m/s; relative speed adds for opposite motion and subtracts for motion in the same direction.
- Work: combined rate is 1/a + 1/b, so days taken = ab/(a + b).
Worked example
₹8,000 is lent at 10% per annum for two years. Simple interest = 8000 × 10 × 2/100 = ₹1,600. Compound interest = 8000[(1.10)² − 1] = 8000 × 0.21 = ₹1,680. The difference is ₹80, which also equals P(R/100)² = 8000 × 0.01 = 80. Recognising the shortcut saves a full calculation and guards against arithmetic slips.
Traps
- Adding successive discounts or percentage changes instead of compounding them.
- Mixing time units: a rate per annum with a period in months, or km/h with metres.
- Averaging two group averages without weighting them by group size.
- Reading "per annum" as per month in interest questions.