Last q working :
1) 40:1 ratio petrol to oil, therefore 5L/40 = 0.125L
2) 4.1 L = 41 parts (40 petrol, 1 oil)
Therefore 1 part is 4.1/41 = 0.1L, so there is 4 L petrol, 0.1 L oil.
4L/25 (25:1 ratio) = 0.16
Hence 0.16-0.1 = 0.06
So he has to add 0.06L or 60 ml of oil