In an enclosure, there are crows as well as cows. If there are 30 heads and 100 legs, what is the ratio of the cows to crows?

Let the number of cows be $x$ and the number of crows be $y$.
Since both cows and crows have exactly $1$ head each:

$$x+y=30\text{— (Equation 1)}$$

Cows have $4$ legs and crows have $2$ legs:

$$4x+2y=100⟹2x+y=50\text{— (Equation 2)}$$

Subtract Equation 1 from Equation 2:

$$(2x+y)-(x+y)=50-30$$

$$x=20$$

Substitute $x=20$ into Equation 1:

$$20+y=30⟹y=10$$

The ratio of cows ($x$) to crows ($y$) is:

$$\frac{x}{y}=\frac{20}{10}=\frac{2}{1}=2:1$$