Mathematics and Quantitative Aptitude
The syllabus shape
Mathematics and Quantitative Aptitude covers number system, percentage, ratio and proportion, average, time–work, time–speed–distance, simple and compound interest, algebra, geometry, mensuration, and data interpretation. WBSSC SLST items are generally school-level but time-bound.
Core rules
- Percentage change: successive changes multiply, not add. Two successive discounts of 20% and 10% give 1 − (0.8 × 0.9) = 0.28, i.e. 28%.
- Average speed for equal distances: harmonic mean, 2xy/(x+y), not (x+y)/2.
- Time–work: convert days to rates. If A does 1/12 and B 1/18 per day, together they do 5/36 per day, so 36/5 = 7.2 days.
- Simple interest: SI = PRT/100; compound interest compounds each period.
- Mensuration: keep units consistent; volume of a cylinder = πr²h.
Worked example
A shopkeeper marks an item at ₹1,000 and gives two successive discounts of 20% and 10%. What is the selling price? After 20%: 1,000 × 0.8 = 800. After 10%: 800 × 0.9 = 720. Single equivalent discount = 28%. The trap is adding 20 + 10 = 30%, which gives 700 - wrong.
Traps
- Adding successive percentages instead of multiplying factors.
- Using the arithmetic mean for average speed.
- Mixing SI and CI for multi-year periods.
- Forgetting that profit percent is calculated on cost price, not selling price.