Alligation

Alligation is an old and practical method of solving arithmetic problems related to mixtures of ingredients. There are two types of alligation: alligation medial, used to find the quantity of a mixture given the quantities of its ingredients, and alligation alternate, used to find the amount of each ingredient needed to make a mixture of a given quantity. Alligation medial is merely a matter of finding a weighted mean. Alligation alternate is more complicated and involves organizing the ingredients into high and low pairs which are then traded off.

Not to be confused with allegation.

Two further variations on Alligation occur : Alligation Partial and Alligation Total (see John King's Arithmetic Book 1795 which includes worked examples.) The technique is not used in schools although it is used still in pharmacies for quick calculation of quantities.

Examples

Alligation medial

Suppose you make a cocktail drink combination out of 1/2 Coke, 1/4 Sprite, and 1/4 orange soda. The Coke has 120 grams of sugar per liter, the Sprite has 100 grams of sugar per liter, and the orange soda has 150 grams of sugar per liter. How much sugar does the drink have? This is an example of alligation medial because you want to find the amount of sugar in the mixture given the amounts of sugar in its ingredients. The solution is just to find the weighted average by composition:

grams per liter

Alligation alternate

Suppose you like 1% milk, but you have only 3% whole milk and ½% low fat milk. How much of each should you mix to make an 8-ounce cup of 1% milk? This is an example of alligation alternate because you want to find the amount of two ingredients to mix to form a mixture with a given amount of fat. Since there are only two ingredients, there is only one possible way to form a pair. The difference of 3% from the desired 1%, 2%, is assigned to the low fat milk, and the difference of ½% from the desired 1%, ½%, is assigned alternately to the whole milk. The total amount, 8 ounces, is then divided by the sum to yield , and the amounts of the two ingredients are

ounces whole milk and ounces low fat milk.

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A general formula that works for both alligation "alternate" and alligation "medial" is the following: Aa + Bb = Cc.

In this formula, A is the volume of ingredient A and a is its mixture coefficient (i.e. a= 3%); B is volume of ingredient B and b is its mixture coefficient; and C is the desired volume C, and c is its mixture coefficient. So in the above example we get: A(0.03) + B(0.005) = 8oz(0.01). We know B = (8oz-A), and so can easily solve for A and B to get 1.6 and 6.4oz, respectively. Using this formula you can solve for any of the 6 variables A,a,B,b,C,c, regardless of whether you're dealing with medial, alternate, etc.

Repeated Dilutions

8 liters are drawn from a cask full of pure wine and is then filled with water. This operation is performed three more times. The ratio of the quantity of wine now left in cask to that of water is 16: 65. How much wine did the cask hold originally? This is an example of a problem that involves repeated dilutions of a given solution. [1]

  • Let Vw be the volume of wine in the cask originally.
  • Let Vt be the total volume of liquid in the cask.
  • Let X be the percentage of wine in the cask originally.

X = original volume of wine/ total volume of liquid in the cask = Vw / Vt

When 8 liters are drawn out, the volume of wine is reduced by 8 X liters while the total volume of liquid remains unchanged as it is re-filled with water.

Let X’ be the new percentage of wine in the cask after this operation

X’ = (original volume of wine – 8 X) / total volume of liquid in the cask

X’ = [Vw – 8 (Vw/ Vt)] / Vt

X’ = X (Vt – 8) / Vt

After 4 such replacement operations, X’’’’ = X [(Vt – 8)/ Vt] ^ 4

From the problem, X’’’’ = 16/ (16 + 65) = 16/ 81

Also, since originally the cask was full of pure wine, X = 1

[(Vt – 8)/ Vt] ^ 4 = 16/ 81

=> Vt = 24 liters

References
  1. https://learningpundits.com/module-view/42-mixtures-and-alligations/1-tips-on-mixtures-and-alligations/
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