The number of chickens raised by Uncle Li is one third of that of ducks. Later, he bought 15 chickens. At this time, the ratio of chickens to ducks is 5:6. How many ducks does Uncle Li raise?
The original chicken: Duck = 1:3 = 2:6
After buying 15 chickens, the ratio becomes 5:6, which increases by 3 / 6, that is, 1 / 2 of the number of ducks,
So there were 30 ducks and 10 chickens
15/(5/6-1/3)=30