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Rule of three inverse proportion

The rule of  three is a method  to  solve  proportions. If  you have three numbers: a, b, c, such that,   ( a / b = c / x), or a: b :: c: x ) you can calculate the unknown value (x).
We have, by the expression: (a : b :: c : x), where the product of the means is equal to the product of the extremes, using the cross products rule, where:
b * c = a * x.
Inverse Proportion
When, the increase of a quantity , causes a decrease in the other one, or by the contrary, a decreasing causing the increase in the other one, this is, an inverse proportion , or the two quantities are inversely proportional.
The expression: a / b = c / x (direct proportion) is now: (b / a = c / x), for the inverse proportion, because, we change the terms of the first equality, where the numerator becomes the denominator and vice versa.
If two workers take 16 days to build 100m of fence, how many days will be spent using four workers?
Problem formulation.
2 workers = 16 days
4 workers =   x days
a / b = c / x )
Since the ratio is inversely proportional, we change the terms of the first equality.
Mathematical Formulation:
b / a = c / x)
x =( a * c) / b )
And now, replacing, the letters, we have:
X = (2 * 16) / 4) = 8 days.
So, as seen above, increasing workers quantities, resulting in decreased days.
How to calculate the Rule of three - inverse proportion.

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