A ratio splits a whole into equal units, so each part can be written as a fraction of the whole. Add the parts to get the total number of units — that becomes the denominator. For parts 2 : 5 there are 7 units in all, so the first part is 2/7 of the whole and the second is 5/7. Remember to simplify the fraction where you can.
Worked example
Problem Two parts are in the ratio 2 : 5. What fraction of the whole is each part?
Add the parts to get the total units: 2 + 5 = 7. This is the denominator.
The first part is 2 units, so it is 2/7 of the whole.
The second part is 5 units, so it is 5/7 of the whole.
Tip A ratio a : b means the whole has (a + b) equal parts.