You have that you need 5 ml of blue water and 15 ml of yellow water to produce green water. That is, you have the following proportion
5/15 = 1/3 "It is necessary 1 ml of blue water and 3 ml of yellow water"
In order to determine how much yellow water you need to obtain green water, by using only 1 ml of blue water. The answer is already found, in the previous simplified proportion 1/3, you have that it is necessary only 1 ml of blue water and 3 ml of yellow water to produce green water.
Hence, it is necessary 3 ml of yellow water