General Ability and Aptitude: Speed with Numerical and Logical Reasoning
General Ability and Aptitude tests reasoning speed and accuracy rather than engineering knowledge: numerical ability, data interpretation, logical and spatial reasoning, and English usage. Engineering settings are only the wrapper for the arithmetic.
Numerical technique
- Percentage change = new/old − 1; a 20% rise then a 20% fall leaves 96%, not 100%.
- Work problems: add rates, never times. If A finishes in a days, its rate is 1/a.
- Speed, time, distance: average speed = total distance / total time, not the mean of the speeds.
- Ratio and proportion: write one equation and cancel units before multiplying.
Worked example
Two pipes fill a tank in 12 and 18 minutes; a drain empties it in 36. Net rate = 1/12 + 1/18 − 1/36 = 3/36 + 2/36 − 1/36 = 4/36 = 1/9 per minute, so the tank fills in 9 minutes. Working in LCM units (36) gives 3 + 2 − 1 = 4 units/min out of 36 - same answer, fewer fractions.
Reasoning and language
- Series: check differences, then ratios, then alternating sub-series before assuming a rule.
- Syllogisms: draw the minimum Venn diagram; only conclusions true in every diagram follow.
- Data interpretation: read axis units and year labels first; totals are often not 100%.
Traps
- "Per cent of" versus "per cent more than".
- Averaging two averages with different bases.
- Answering the wrong quantity - many items ask for the value after a change, not the change.
- Long passages: read the question stem before the text.