If 4 liters of red is equal to 2 parts red, we can see that 1 part is equal to 2 liters since 4 devided by 2 is equal to 2. Now we know that 1 part is equal to 2 liters, now we plug in the mising values with what we know. Since it calls for 3 parts cream, and that 1 part is equal to 2 liters, we can conclude that the total amount of cream needed would be 6 liters since 3 times 2 equals 6. This is the same for the 1 part blue. Since we know that 1 part is equal to 2 liters, all we need to do is to plug in the missing vales, which would be 2 liters of blue. So to wrap things up, you will need 6 liters of cream and 2 liters of blue, or 8 liters in total.
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