Question

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?

Answer 1

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

Answer: 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.