Liz makes a large amount of purple paint by mixing red and blue paint in the ratio of 3: 7. red paint costs £20 for a 3 litre tin. blue paint costs £30 per 3 litres. liz wants to make 30 liters of purple paint. work out how much it will cost her.

3 liter of red paint and 7 liter of blue paint mixed together makes 10 liters of purple paint.
to make 30 liters of purple paint, because 30 is 3 times 10, you will need 3 times 3 liters of red and 3 times 7 liters of bule, which is 9 and 21 respectively.
9 liters of red paint is 3 tins, so the cost if 60 dollars, and 21 liters of blue paint is also 7 tins, the cost is 7*30=210
the total is 270

All you have to do is multiply