Q1.
What is the total volume of this purple paint? (Red : Blue : White :: 2 : 3 : 5, with 4 litres of red, 6 litres of blue and 10 litres of white.)
Answer
Add the three amounts.
4 + 6 + 10 = 20 litres
Why it happens: There is a quicker route that does not need the individual amounts at all. The ratio 2 : 3 : 5 has 2 + 3 + 5 = 10 parts in total, and white is 5 of them — exactly half the mixture. So if the white paint is 10 litres, the whole batch must be 20 litres.
Tip: Adding the terms of a ratio to get the “number of parts in the whole” is the single most useful move in this chapter. It is what Section 3.4 turns into a formula.