8 + 1 + 1 = 10, so there are 10 parts to the ratio. 3.5 litres divided by 10 would mean that each part of the ratio is worth 0.35 litres. The colour is worth 8 parts of the ratio. 0.35 litres multiplied by 8 would give 2.8 litres, meaning 2.8 litres of colour are required. Fiona has 2.9 litres of colour, which is more than enough to make 3.5 litres of paint mix.
