Mental Ability and Quantitative Aptitude: Formula Plus Logic
Mental Ability / Reasoning and Quantitative Aptitude in UPSSSC VDO mixes logic (series, coding-decoding, blood relations, direction sense, syllogism, seating) with arithmetic (percentage, ratio, average, profit-loss, SI/CI, time-work, speed, mensuration). The scoring strategy is to identify the pattern or formula in the first few seconds.
Core rules
- Percentage change: successive changes use a + b + ab/100. Profit% = (SP − CP)/CP × 100.
- Speed: km/h × 5/18 = m/s. Time to cross a pole = train length / speed.
- Average = sum / count. Work: if A takes a days and B takes b days, together they take ab/(a+b) days.
- SI = PRT/100; CI = P(1 + r/100)^t − P.
- Blood relations: draw a small family tree and mark gender; do not assume from names.
- Direction sense: fix north and track left/right turns; final displacement uses Pythagoras when turns are at right angles.
Worked example
"A number is increased by 20% and then decreased by 20%. What is the net change?" Take 100: after a 20% increase it becomes 120; a 20% decrease on 120 gives 96. Net change = −4%, i.e. a 4% decrease. The formula gives 20 + (−20) + (20 × −20)/100 = −4. The base changes after the first step, which is why the answer is not zero.
Traps
The second percentage applies to the new value, not the original. A 10% loss followed by a 10% gain also gives a net loss. In syllogism, do not convert "some A are B" into "some B are A" unless the option is logically valid. In direction questions, distinguish facing direction from movement direction.