White paint costs £2.80 per litre. Blue Paint costs £3.50 per litre. White paint and blue paint are mixed in the ratio 3:2 Work out the cost of 18 litres of the mixture.

The mixture has a ratio of 3:2, white to blue paint. 
3+2=5 so there are 5 parts to the mixture, 3 white and 2 blue.
We know the cost of 1 litre of each, so first we can work out the cost of 1 litre of the mixture.
⅗ of the mixture is white, so we need to work out what ⅗ of a litre of white paint costs, which is ⅗ x 2.80 .
⅖ the mixture is blue, so we need to work out what ⅖ of a litre of white paint costs, which is ⅖ x 3.50 .
So the cost of 1 litre of the mixture is: (⅗ x 2.80) + (⅖ x 3.50) 

Breaking this down, we can figure out ⅕ of 2.80 first: 2.80 ÷ 5 = 0.56
And then ⅗ :  0.56 x 3 = (0.5 x 3) + (0.06 x 3) = 1.5 + 0.18 = 1.68
Next finding ⅕ of 3.50:  3.50 ÷ 5 = 0.70
And then ⅖ :  0.70 x 2 = 1.40
Meaning the cost of a full litre of the mixture is:  1.68 + 1.40 = £3.08 

To work out 18 litres we need to calculate: 18 x 3.08
Breaking this down again this is: (10 x 3.08) + (8 x 3.08)
= 30.80 + (8 x 3) + (8 x 0.08) 
= 30.80 + 24 + 0.64 = 55.44
So the cost for 18 litres of the mixture is £55.44