SSC CGL percentage — free lecture, notes and practice questions

Per cent means 'per hundred'. Writing 25% is just another way of writing 25/100, or the fraction 1/4. Every percentage question becomes easier the moment you stop calculating and start converting: x% of y is simply (x/100) × y, and the order does not matter, so 16% of 25 equals 25% of 16 equals 4.

SSC rarely asks you to compute a percentage in isolation. It asks about change — a price rises, a population falls, income increases while expenditure stays fixed. So always identify the base first: the number the change is measured against. Most wrong answers in this chapter come from taking the wrong base, not from bad arithmetic.

• x% of y = (x/100) × y, and x% of y = y% of x.
• Percentage change = (change ÷ original value) × 100.
• Always write down which number is the base before solving.

Toppers do not calculate 12.5% of 480 — they know 12.5% is 1/8 and divide instantly. Memorising the table below is the fastest single hour you can invest in SSC Maths.

• 1/2 = 50%, 1/3 = 33⅓%, 1/4 = 25%, 1/5 = 20%, 1/6 = 16⅔%.
• 1/7 = 14²⁄₇%, 1/8 = 12.5%, 1/9 = 11⅑%, 1/10 = 10%, 1/11 = 9¹⁄₁₁%.
• 1/12 = 8⅓%, 1/15 = 6⅔%, 1/16 = 6.25%, 1/20 = 5%, 1/25 = 4%.

When a quantity changes twice in a row, the changes never simply add. If a value changes by a% and then by b%, the net change is a + b + (ab/100), using a minus sign for a decrease. A 20% rise followed by a 20% fall gives 20 − 20 − 400/100 = −4%, so the value ends up 4% lower — a classic SSC trap.

The same formula answers discount-on-discount questions and 'length increased, breadth decreased' area problems, which is why it appears in some form in almost every paper.

• Net change = a + b + (ab/100).
• Two successive 10% discounts equal a single 19% discount, not 20%.
• For three changes, apply the formula twice.

If a value is increased by x%, the percentage decrease needed to bring it back to the original is (x ÷ (100 + x)) × 100. If it is decreased by x%, the increase needed to restore it is (x ÷ (100 − x)) × 100. Salary, price and consumption questions are almost always this pattern in disguise.

A worked example: if the price of rice rises by 25%, by how much must consumption fall so expenditure stays the same? Answer: 25/(125) × 100 = 20%. Notice that assuming the answer is also 25% is the mistake the question is designed to catch.

• Increase x% → decrease needed = 100x/(100 + x).
• Decrease x% → increase needed = 100x/(100 − x).
• Fraction shortcut: a 25% rise is +1/4, so the reversal is −1/5 = 20%.

Pick one educator and finish their full percentage playlist rather than sampling five channels. Watch the concept video at 1x, then rewatch the solved-example portion at 1.25x while solving alongside on paper. Finish with a previous-year set of 30 questions in 20 minutes.

• Day 1: concept + fraction table.
• Day 2: successive change and reversal problems.
• Day 3: application in profit-loss and interest.
• Day 4: 50 previous-year questions, timed.