Quantitative Aptitude / Numerical Ability
Quantitative Aptitude / Numerical Ability checks speed and accuracy with numbers in commercial contexts. The BoB Office Assistant section typically includes simplification, number series, percentage, ratio and proportion, average, profit and loss, simple and compound interest, time-work, speed-distance, mensuration, and data interpretation.
Core rules
- Follow BODMAS/PEMDAS; simplify brackets first, then powers/roots, division/multiplication, addition/subtraction.
- Percentage change = (new − old)/old × 100; successive changes multiply factors, not add percentages.
- In ratio chains, make the common term equal (e.g., A:B = 2:3 and B:C = 4:5 gives A:B:C = 8:12:15).
- Work: if A finishes in x days and B in y days, together they take xy/(x+y) days; use LCM of days as total work.
- Simple interest = P × R × T/100; compound interest = P[(1+R/100)^T − 1].
Worked example
A shopkeeper marks an article 40% above cost price and allows a 25% discount. Find profit percent. Let CP = 100. Marked price = 140. Discount is on MP: SP = 140 × 0.75 = 105. Profit = 105 − 100 = 5, so profit% = 5/100 × 100 = 5%. The common error is taking 25% of CP, which gives 140 − 25 = 115 and a wrong 15%.
Traps
Applying discount to cost price; adding successive percentage changes; confusing CI with SI for more than one year; using the wrong base in average or ratio questions; not checking units in speed-distance and mensuration.