$30000$

To calculate $120\times 250$ quickly, we use the rule $250=\frac{1000}{4}$.

1. First, divide $120$ by $4$: $120÷4=30$.
2. Next, multiply the result by $1000$: $30\times 1000=30000$.

Therefore, $120\times 250=30000$.

$120000$ is incorrect; it's the result of multiplying by $1000$ without dividing by $4$.

$60000$ is incorrect; it results from dividing by $2$ instead of $4$.

$3000$ is incorrect; it results from multiplying by $100$ instead of $1000$.

Ok