More of a quantitative section is percentages than the topic list suggests. Profit and loss is percentages of cost price. Simple interest is percentages per year. Discount, population growth, depreciation and most data-interpretation questions are percentages wearing different words.
So the time spent getting fast here pays back several times over, and the single most useful habit is to stop computing percentages and start multiplying by factors: a 20% rise is not "find 20% and add it", it is "times 1.2".
Try each one before opening the answer. A solution you have read is not a solution you can produce under time pressure, which is the only kind that counts.
Q1 A shopkeeper raises a price by 20% and later cuts the new price by 20%. What is the overall change?
A fall of 4%.
Multiply the factors rather than adding the percentages: 1.20 × 0.80 = 0.96, which is 4% below the original. The formula gives the same thing: 20 + (−20) + (20 × −20)/100 = −4. The cut is bigger in money terms than the rise because it is 20% of a larger number.
Q2 If A’s salary is 25% more than B’s, by what percentage is B’s salary less than A’s?
20%.
Let B = 100, so A = 125. B is 25 less than A, and 25/125 = 0.2, so 20%. The standard formula 100x/(100 + x) gives 100 × 25 / 125 = 20. The answer is not 25% because the comparison is now against the larger number.
Q3 In an election between two candidates, the winner got 56% of the votes and won by 1680 votes. How many votes were cast?
14,000.
The loser got 44%, so the margin is 56 − 44 = 12% of the total. That 12% is 1680 votes, so the total is 1680 × 100 / 12 = 14,000. The mistake to avoid is dividing by 56 rather than by the margin.