How do you write a ratio as a fraction?

If, in a bag, there are 2 red discs and 5 blue discs then:

R : B : Total

2 : 5 : 7

dividing by the total, we have

Red Blue Total

2/7 : 5/7 : 1

Q: what fraction of the number of discs in the bag is red?

A: 2/7

However, if the question is:

Q: write the number of red discs as a fraction of the number of blue discs.

A: 2/5

It depends on exactly what they are asking for.

If they are asking that you express one part of the ratio as a fraction then you find the total number of parts by adding them together and this is the denominator of your fraction and the numerator is the part you've been asked about.

Example Ratio 3: 5, Express 1st part as a fraction.

Total no of parts is 3+ 5 = 8

Fraction is 3/8

If they are asking that you express one part of the ratio as a fraction of the other part of the fraction the the denominator is the "other part" and the numerator is the first part.

Example Ratio 3:5, Express 1st part as a fraction of the 2nd part

Fraction is 3/5

A ratio is said to be is the simplest form or in the lowest term if two quantities of a ratio (i.e., antecedent and consequent) have no common factor (i.e. antecedent and consequent are co-prime) other than 1 (or their HCF is 1.) For example: (i) The ratio between 36 kg and 24 kg = (36 kg)/(24 kg)

If you have a ratio of blue : red at 3 : 2. The whole amount is 5, so that is the denominator. Blue would be 3/5 and red would be 2/5.