Question

I don't get how to do this.

Answer 1

`426\:\mathrm{chickens}`

Explanation:

Since a duck has two legs, a pig has four legs, and a chicken has two legs, we can write the following system of equations:

`\begin{cases}d+p+c=639,\\d=2p,\\c=3d\end{cases}`, where
`d` is the number of ducks on the farm,
`c` is the number of chickens, and
`p` is the number of pigs.

Substituting, we get:

`d+(1)/(2)d+3d=639,\\4.5d=639,\\d=\boxed{142}`

Now substitute
`d=142` into
`c=3d` to get the number of chickens:

`c=3(142)=\boxed{426\:\mathrm{chickens}}`

Answer 2

426 chickens

Explanation:

let the pigs = x

the ducks = 2×x = 2x

the chickens = 3×(2x) = 6x

x +2x +6x = 639

9x=639

x = 639/9 = 71

the chickens = 6x = 6×71 = 426