Section A: Arithmetic & Numerical Ability
Arithmetic & Numerical Ability in DSSSB PGT tests school-level quantitative reasoning: number system, percentages, ratio-proportion, averages, profit and loss, simple and compound interest, time-work, time-speed-distance, mensuration, and data interpretation. Questions reward clean method and unit discipline.
Core rules and concepts
- Percentage: always identify the base. A 20% increase followed by a 20% decrease gives a net 4% decrease, because 1.20 × 0.80 = 0.96.
- Ratio: convert to a common unit and use a multiplier k.
- Profit/loss: use CP as base; SP = CP × (100 ± profit/loss%)/100.
- SI and CI: SI = PRT/100; CI compounds annually unless stated otherwise.
- Time-work: total work = LCM of days; individual rates add.
- Speed: convert km/h to m/s by multiplying by 5/18.
- Mensuration: keep all dimensions in the same unit before calculating area or volume.
Worked example
A can finish a job in 12 days and B in 18 days. Working together, how long will they take? Reasoning: take total work as LCM(12,18)=36 units. A does 3 units/day, B does 2 units/day. Together they do 5 units/day. Time = 36/5 = 7.2 days, i.e. 7 days 4.8 hours. Do not average 12 and 18 to get 15; rates add, not times.
Traps
- Using the wrong base in successive percentage changes.
- Mixing SI and CI for multi-year periods.
- Forgetting to convert minutes to hours in speed questions.
- Treating 7.2 days as 7 days 2 hours; 0.2 day is 4.8 hours.
- Using the wrong formula for curved surface area versus total surface area.