Ratio properties

Very Important

Equivalence of ratios:  $$a : b = ak : bk$$, where $$k$$ is a constant.

$$a,b,c,d$$ four terms are in proportion.

Invertendo: $$a:b=c:d$$ then $$b:a=d:c$$

Alternendo: $$a:b=c:d$$ then $$a:c=b:d$$

Componendo: $$a:b=c:d$$ then $$\dfrac{a+b}{b}=\dfrac{c+d}{d}$$

Dividendo: $$a:b=c:d$$ then $$\frac{a-b}{b}=\frac{c-d}{d}$$

Componendo-Dividendo: $$a:b=c:d$$ then $$\dfrac{a+b}{a-b}=\dfrac{c+d}{c-d}$$

Question 1

The number of students in a school is a prime number less than 500. Which of the following can be a possible ratio of the number of boys to girls?

Question 2

Two farms A and B have 10 sheep each in January 2000. At the start of each year, 105 sheep are added to A and the number of sheep in B is doubled. What is the ratio of the number of sheep in A and B in February 2006?

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Ratio properties

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