How to see it
A ratio compares parts to parts. One to three means for every one red there are three blue. Count the parts to find the whole: one plus three is four. So red is one part out of four altogether. Part to part, or part to whole, always check which the question wants.
A worked example
Equivalent ratios: 1 to 3 is the same as 2 to 6, or 5 to 15. Multiply both sides by the same number. And the fraction red is 1 over 4.
The common mistake: reading a part-to-part ratio as a fraction of the whole
The trap is writing one third. That would be one red for every two others. One third of the counters being red is the ratio one to two. Add the parts first.
Try one
The ratio of boys to girls is 2 to 5. What fraction are girls?
- A5/2
- B2/7
- C5/7
- D5/3
Show the answerHide the answer
Right: C, 5/7
Two plus five is seven parts, five are girls.