Two tank contain petrol and diesel. In the first tank the ratio of petrol to diesel is 7:4 and in the second tank the ratio is 4 : 5. A third tank of capacity 38L is filled with these two tanks so that the mixture contains the ratio of petrol to diesel is 5 : 4. How many litres are taken from the second tank?

Let x litres from first tank (7:4) and y litres from second tank (4:5). Target ratio 5:4 means 5/9 petrol. From first: 7/11 petrol, from second: 4/9 petrol. Using alligation: (7/11 - 5/9):(5/9 - 4/9) = (63-55)/99 : 1/9 = 8/99 : 11/99 = 8:11. So x:y = 11:8. Total = 11x + 8x = 19x = 38, x=2. From second tank: 8×2 = 16 litres.