Quantitative Aptitude
What this domain tests
Quantitative Aptitude checks numerical fluency in percentages, ratio and proportion, averages, profit and loss, simple and compound interest, time-work, speed-distance, mensuration and data interpretation. The JSA level usually avoids heavy algebra; the winning method is to convert words into one clean equation and use unit values.
Core rules
- Percentage change: successive changes of +a% and -b% give net change a - b - ab/100, with signs applied.
- Profit and loss is always calculated on cost price unless the question says otherwise; discount is on marked price.
- Average = total/sum count; if every item changes by k, the average changes by k.
- Time-work: use one day's work as a fraction; if A finishes in x days, A's one-day work is 1/x.
- Speed = distance/time; for equal distances, average speed is 2xy/(x+y), not (x+y)/2.
- Mensuration: keep units consistent; area is square units, volume cubic units.
Worked example
A shopkeeper marks an item at ₹1,000 and gives a 20% discount. He still makes a 25% profit on cost. What is the cost price? Selling price = 1000 × (1 - 0.20) = ₹800. Profit 25% means SP = 1.25 × CP, so CP = 800 / 1.25 = ₹640. Check: 25% of 640 = 160; 640 + 160 = 800. The discount is on the marked price, while profit is on cost price - the two bases are different.
Traps that cost marks
Do not calculate profit on selling price by habit. In compound interest, interest is added to principal each year; simple interest is not. In time-work, do not average completion times directly. In percentage, a 20% increase followed by a 20% decrease is a 4% decrease, not zero. Convert minutes to hours before mixing with km/h.