How many liters of 40% oil dressing should be mixed with 4 liters of a dressing that is 25% oil to get a dressing that is 30% oil?

Volume of first oil = A, volume of second oil = B, volume of mixture = M
A = ?, B = 4l, M = A + B

40%A + 25%B = 30%M
40%A + 25%4l = 30%(A+4l)
40%A + 25%4l = 30%A + 30%4l
40A + 100l = 30A + 120l
40A - 30 A = 120l - 100l
10A = 20l
A = 2l

2 litres of 40% oil dressing.

Also note: That's an approximation, because the volumes are not strictly additive. For example: mixing 50ml pure ethanol with 50ml water will give you about 95ml of mixture. To get an accurate answer, you'd have to measure the volume of the final mixture and then divide your total oil content by that.

the law of definite proportions states samples of a compound will always contain the same proportion of elements by mass. in other words, a given compound will always contain the same elements in the same proportions by mass.the law of multiple proportions states when two elements combine with each other to form more than one compound, then the weights of one element that combine with a fixed weight of the other are in a ratio of small whole numbers.