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Aptitude for placements

One 60-second card per skill, in learning order. 10 of 10 cards ready.

Percentages 2/2Profit and loss 2/2Ratio and proportion 2/2Time and work 2/2Number series 2/2

In this module

1Percentages2 of 2 cards2Profit and loss2 of 2 cards3Ratio and proportion2 of 2 cards4Time and work2 of 2 cards5Number series2 of 2 cards

Topic 1

Percentages

Percent of a number · Percent increase and decrease

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60-second card

Percent of a number

Percent means 'out of 100'. To find x% of a number, multiply by x and divide by 100.

  • 20% of 450 = 450 × 20 ÷ 100 = 90.
  • To find what percent a is of b: a ÷ b × 100.
  • 10% is easy: move the decimal one place. Build other percents from it.

Example: Ravi scored 42 out of 60. 42 ÷ 60 × 100 = 70%.

Remember: Part ÷ whole × 100 gives the percent.

Common mistake: Dividing by the wrong number. The 'of' number (the whole) goes at the bottom.

60-second card

Percent increase and decrease

Percent change compares the change with the starting value.

  • Change % = (new − old) ÷ old × 100.
  • A rise of 20% then a fall of 20% doesn't get you back: you end 4% lower.
  • Growth each year multiplies: 10% a year for 2 years is × 1.1 × 1.1.

Example: A bag goes from ₹800 to ₹920. Change = 120. 120 ÷ 800 × 100 = 15% increase.

Remember: Always divide by the old (starting) value.

Common mistake: Dividing by the new value, which gives a different, wrong answer.

Topic 2

Profit and loss

Profit and loss percent · Discounts

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60-second card

Profit and loss percent

Profit or loss percent is always worked out on the cost price (CP).

  • Profit = SP − CP; loss = CP − SP.
  • Profit % = profit ÷ CP × 100.
  • SP for a 15% profit = CP × 1.15; for a 20% loss = CP × 0.8.

Example: Bought for ₹40, sold for ₹50. Profit = 10. 10 ÷ 40 × 100 = 25% profit.

Remember: Percent of the cost price, not the selling price.

Common mistake: Dividing the profit by the selling price.

60-second card

Discounts

A discount is worked out on the marked price (MP), the price on the tag.

  • Selling price = MP − discount. 15% off ₹2,000 = ₹1,700.
  • Two discounts one after another multiply: 10% then 20% = 0.9 × 0.8 = 0.72, so 28% off.
  • Buy 3, get 1 free = 1 free in 4 = 25% off.

Example: Marked ₹1,500, 20% off: 1,500 × 0.8 = ₹1,200.

Remember: Successive discounts multiply; they don't add.

Common mistake: Adding 10% and 20% to get 30%. The real total discount is 28%.

Topic 3

Ratio and proportion

Simplifying ratios · Sharing in a ratio

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60-second card

Simplifying ratios

A ratio compares quantities. Simplify it by dividing every part by the same number.

  • 24 : 36 → divide by 12 → 2 : 3.
  • To join a : b and b : c, make the b parts equal first.
  • Proportion: a : b = c : d means a × d = b × c.

Example: a : b = 2 : 3 and b : c = 4 : 5. Make b = 12: a : b = 8 : 12, b : c = 12 : 15. So a : c = 8 : 15.

Remember: Divide all parts by their highest common factor.

Common mistake: Adding the same number to both parts. That changes the ratio.

60-second card

Sharing in a ratio

To share an amount in a ratio, add the parts, then give each person their share of the total.

  • Total parts = add the numbers in the ratio.
  • One part = amount ÷ total parts.
  • Each share = their number × one part.

Example: Share ₹1,200 in 3 : 5. Parts = 8, one part = 150. Shares: ₹450 and ₹750.

Remember: Find one part first; everything else follows.

Common mistake: Dividing the amount by the ratio numbers themselves instead of by their total.

Topic 4

Time and work

Work per day · Working together

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60-second card

Work per day

If a job takes n days, you do 1/n of it each day. Working with rates makes these sums easy.

  • 10 days for a job → 1/10 of the job per day.
  • Work done = rate × time.
  • More workers, fewer days: workers × days stays the same.

Example: 6 workers take 10 days: 60 worker-days. 15 workers need 60 ÷ 15 = 4 days.

Remember: Turn every time into a rate (job per day).

Common mistake: Averaging the days instead of adding the rates.

60-second card

Working together

When people work together, add their rates. The time is 1 ÷ (total rate).

  • A: 1/12 per day, B: 1/6 per day.
  • Together: 1/12 + 2/12 = 3/12 = 1/4 per day, so 4 days.
  • A leak or drain has a negative rate: subtract it.

Example: Tap fills in 4 h, drain empties in 6 h. Rate = 1/4 − 1/6 = 1/12 per hour, so 12 hours.

Remember: Add rates, never add days.

Common mistake: Adding the days (12 + 6 = 18). Working together must be faster than either alone.

Topic 5

Number series

Adding patterns · Other patterns

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60-second card

Adding patterns

In an adding pattern, the same number is added (or taken away) each time.

  • 7, 12, 17, 22: add 5 each time, so next is 27.
  • The nth term = first + (n − 1) × difference.
  • If the differences themselves grow steadily (5, 10, 15), look at the second differences.

Example: 3, 7, 11, 15, ... difference 4. 10th term = 3 + 9 × 4 = 39.

Remember: Write the differences between neighbours first.

Common mistake: Guessing from the first two numbers only. Check the pattern across all of them.

60-second card

Other patterns

If the gaps aren't equal, try multiplying, squares and cubes, or two patterns taking turns.

  • Multiplying: 3, 6, 12, 24 (× 2 each time).
  • Squares 1, 4, 9, 16 and cubes 1, 8, 27, 64.
  • Alternating: 7, 10, 8, 11, 9 is +3, −2, +3, −2.

Example: 3, 5, 9, 17, 33: each is double the last minus 1, so next is 65.

Remember: Check differences, then ratios, then squares, then alternating patterns.

Common mistake: Stopping at the first pattern that fits two numbers without checking the rest.

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