Toula observed the number of times she saw a red car, white car and blue car pass by an intersection. She kept a tally of these colours and ignored all other colours. Out of 219 cars, there were 13 more white cars than red cars, and the number of blue cars was 2/3 the number of white cars. What was the most popular colour, and how many did Toula see of that colour?

Answer: white; 87 (want to know who to work this question out)

Question

Toula observed the number of times she saw a red car, white car and blue car pass by an intersection. She kept a tally of these colours and ignored all other colours. Out of 219 cars, there were 13 more white cars than red cars, and the number of blue cars was 2/3 the number of white cars. What was the most popular colour, and how many did Toula see of that colour? 

Answer: white; 87 (want to know who to work this question out)

gw2011 · Answer

You need to set up the problem in terms of one of the unknowns.  In this case the white car is the common unknown.  So, you can set up the following:

Let w=white car
Let r=red car=w-13 since there are 13 fewer red cars than white ones
Let b=blue car=2/3w since the blue cars are 2/3 of the white ones

Now use the above three set-ups in an equation that equals the total number of cars and solve.