how to calculate ratios? simplify plz?

Answer 1

A ratio is simply a fraction such as a/b or 5/2 or 3/7.
If we say that the ratio of boys to girls is 1/3, then for every boy there is 3 girls. There really is no calculating to be done.

A proportion is a statement of equality between two ratios.
a/b = c/d is a proportion. It means that bc=ad.
1/2 = 3/6 is a true proportion because 1*6 = 3*2.

If you are given x/2 = 4/9 then 2*4 = 9x so x = 8/9
If you wre given 1/x = x/9, then x^2 = 9 so x = +/- 3

Answer 2

Just cross multiply and divide.
Example:
12 girls per 20 students = x girls per 60 students
12/20=x/60
20x=12(60)
20x=720
x=36