Mixture & Alligation

The Art of Weighted Averages. Master the 'See-Saw' method for balancing and the 'Cross Method' for instant ratios.

1. The 'See-Saw' (Weighted Average)

Visualizing Balance.

  • Concept: If 10kg is at 20Rs and 20kg is at 50Rs.
  • Balance: Weight Ratio 1:2. The balance point shifts towards 50. Mean = 4040.

Example:

Q: Avg of Class A (20 students) is 60. Class B (30 students) is 70.
Solution: Ratio 2:3. Gap 10. Break Gap in 3:2 inverse. 3/53/5 of 10 is 6. 60+6=6660+6=66.

2. The 'Cross' (Alligation Rule)

To find Ratio when Mean is given.

  • Rule: Diagonally subtract. тИгDтИТMтИг|D-M| and тИгMтИТCтИг|M-C|.
  • Important: All units must be same (e.g., all CP or all SP).

Example:

Q: Rice @ 30 mixed with Rice @ 40 to get 33.
Solution: 30 ... 40
..33..
Diff: 7 : 3. Ratio 7:3.

3. The 'Poison' Formula (Replacement)

Repeated dilution logic.

  • Formula: Rem=Initial├Ч(1тИТOutCapacity)nRem = Initial \times (1 - \frac{Out}{Capacity})^n.
  • Use for pure liquid left after n operations.

Example:

Q: 100L Milk. 10L replaced with water. 2 times.
Solution: Rem=100(1тИТ10/100)2=100(0.9)2=81LRem = 100 (1 - 10/100)^2 = 100(0.9)^2 = 81L.

4. Profit on Mixture (Water is Free)

If mixture sold at Cost Price of Milk yields Profit.

  • Hack: Profit % is exactly the quantity of Water added (if Water is free).
  • Rule: If 20% Profit, Ratio Water:Milk = 20:100 = 1:5.

Example:

Q: Sold at CP, gained 25%. Find Water:Milk.
Solution: Gain 25%. Values: Water=25, Milk=100. Ratio 1:4.

5. Ratio Shift (Adding One Component)

When only water is added, Milk remains constant.

  • Method: Equate the constant part in both ratios.
  • If Milk:Water changes 4:1тЖТ4:34:1 \to 4:3. Milk is same (4). Water increased by 2 units.

Example:

Q: Mix 4:1. Add 10L water, becomes 4:3.
Solution: Milk constant. Water gap 3тИТ1=23-1 = 2 units. 2u=10LтЖТ1u=5L2u=10L \to 1u=5L.