Viral Maths — Ch09: Percentage

📌 What This Chapter Covers

Fraction–percentage conversion is the foundation of this chapter. Link every percentage to its fractional form to make multiplication and division faster.
There are twelve named types, all based on the same core idea: convert the percentage to a fraction and then multiply.
The Dal Badlu Approach is a special reversal trick that simplifies difficult-looking percentage questions.

⚡ QUICK RECALL

The core formula of the chapter is a% of b = (fractional form of a%) × b. The Fraction–Percentage Table in Tab 2 is the backbone of the chapter and should be memorized first.

🗂️ Chapter Index

Tab	Content	Fractions Used
2	Fraction-Percent Reference Table	All key values
3	Type 1–2: 25%,50%,10%,20%	1/4, 1/2, 1/10, 1/5
4	Type 3–4: 40%,60%,75%,80%	2/5, 3/5, 3/4, 4/5
5	Type 5: 16.66%,33.33%	1/6, 1/3
6	Type 6–7: 6.66%,6.25%,12.5%,14.28%	1/15, 1/16, 1/8, 1/7
7	Type 8–9: 37.5%,7.14%,62.5%,9.09%	3/8, 1/14, 5/8, 1/11
8	Type 10,12: 11.11% & derived %s	1/9, multiples of known fractions
9	Type 11: Dal Badlu Approach	a% of b = b% of a
10	Master Table	All types summarized

🔢 Fraction ↔ Percent — Complete Reference Table

Fraction	%	Fraction	%	Fraction	%
1/2	50%	1/7	14.28%	3/8	37.5%
1/3	33.33%	2/7	28.57%	5/8	62.5%
2/3	66.67%	3/7	42.85%	7/8	87.5%
1/4	25%	4/7	57.14%	1/9	11.11%
3/4	75%	5/7	71.43%	2/9	22.22%
1/5	20%	6/7	85.71%	3/9	33.33%
2/5	40%	1/8	12.5%	4/9	44.44%
3/5	60%	1/6	16.67%	1/10	10%
4/5	80%	5/6	83.33%	1/11	9.09%
		1/12	8.33%	1/14	7.14%
		1/15	6.66%	1/16	6.25%

⚡ QUICK RECALL

This table is used in both Chapter 5 (Division, Broken Heart Approach) and Chapter 9. Memorizing it perfectly is essential.

Type 1 — Finding 25% and 50% of Numbers

25% = 1/4, 50% = 1/2

25% of 48 = 1/4 × 48 = 12 50% of 642 = 1/2 × 642 = 321

Type 2 — Finding 10% and 20% of Numbers

10% = 1/10, 20% = 1/5

10% of 450 = 1/10 × 450 = 45 20% of 655 = 1/5 × 655 = 131

⚡ QUICK RECALL

10% = shift the decimal one place left; 20% = double 10%; 25% = ÷4; 50% = ÷2. These are the most common exam percentages and should be recalled instantly.

Type 3 — Finding 40% and 60% of Numbers

40% = 2/5, 60% = 3/5

40% of 65 = 2/5 × 65 = 26 60% of 125 = 3/5 × 125 = 75

Type 4 — Finding 75% and 80% of Numbers

75% = 3/4, 80% = 4/5

75% of 48 = 3/4 × 48 = 36 80% of 225 = 4/5 × 225 = 180

⚡ QUICK RECALL

For the /5 family (20%, 40%, 60%, 80%), divide by 5 and multiply by the numerator. For the /4 family (25%, 75%), divide by 4.

Type 5 — Finding 16.66% and 33.33% of Numbers

16.66% = 1/6, 33.33% = 1/3

16.66% of 246 = 1/6 × 246 = 41 33.33% of 810 = 1/3 × 810 = 270

⚠ EXAM TRAP

Both 16.66% and 33.33% belong to the Point% family. Their exact fractions come from repeating decimals (1/6=0.1666..., 1/3=0.3333...). Although an exam may show a rounded percentage, use the EXACT fraction.

Finding 6.66% and 6.25% of Numbers

6.66% = 1/15, 6.25% = 1/16

6.66% of 225 = 1/15 × 225 = 15 6.25% of 128 = 1/16 × 128 = 8

Finding 12.5% and 14.28% of Numbers

12.5% = 1/8, 14.28% = 1/7

12.5% of 488 = 1/8 × 488 = 61 14.28% of 154 = 1/7 × 154 = 22

⚡ QUICK RECALL

1/16=6.25% | 1/15=6.66% | 1/8=12.5% | 1/7=14.28%. Do not confuse these values; their denominators are very close.

Finding 37.5% and 7.14% of Numbers

37.5% = 3/8, 7.14% = 1/14

37.5% of 64 = 3/8 × 64 = 24 7.14% of 350 = 1/14 × 350 = 25

Finding 62.5% and 9.09% of Numbers

62.5% = 5/8, 9.09% = 1/11

62.5% of 32 = 5/8 × 32 = 20 9.09% of 473 = 1/11 × 473 = 43

⚡ QUICK RECALL

7.14% = half of 14.28% (both from 1/7 family: 1/14 and 1/7). 9.09% comes from 1/11 — connects to Ch05 Division Type-7 (÷11 family).

Type 10 — Finding 11.11% of Numbers

11.11% = 1/9

11.11% of 819 = 1/9 × 819 = 91

Type 12 — Deriving New Percentages from Known Fractions

Multiply a known base fraction to get related percentages

Example: 22.22% (derived from 11.11%)

11.11% = 1/9 So 22.22% = 2 × (1/9) = 2/9

Example: 7.14% (derived from 14.28%)

14.28% = 1/7 So 7.14% (half of 14.28%) = 1/14

⚡ QUICK RECALL

If a percentage's double, half or multiple is related to a known fraction, derive the new fraction in the same proportion. There is no need to memorize another table.

Type 11 — Dal Badlu Approach (Reversal Trick)

Rule: a% of b = b% of a — swap which number carries the % symbol

Example: 24% of 25

Rewrite as: 25% of 24 Step 1: 25% = 1/4 Step 2: 1/4 × 24 = 6

⚠ EXAM TRAP

“Dal Badlu” means swap the values. When the percentage value is awkward, such as 24%, but the other number forms a convenient fraction, such as 25=1/4, swap them. The answer remains the same while the calculation becomes easier.

⚡ QUICK RECALL

112% of 12.5 = 12.5% of 112 = (1/8)×112 = 14. In this way, apparently difficult percentage questions become simple immediately.

📋 Master Table — All Percentage Types

%	Fraction	%	Fraction
10%	1/10	37.5%	3/8
20%	1/5	62.5%	5/8
25%	1/4	87.5%	7/8
50%	1/2	11.11%	1/9
40%	2/5	22.22%	2/9
60%	3/5	9.09%	1/11
75%	3/4	8.33%	1/12
80%	4/5	7.14%	1/14
16.66%	1/6	6.66%	1/15
33.33%	1/3	6.25%	1/16
12.5%	1/8	14.28%	1/7

🔑 Special Techniques Recap

Dal Badlu Approach: a% of b = b% of a (swap when the other number gives a nicer fraction)
Derived %: multiples/halves of known fractions give new percentages instantly (e.g. 22.22% = 2×11.11%)