We know:
$$60$$ seconds = $$1$$ minute. So multiplying something by:
$$ \dfrac{1\ min}{60\ sec} $$ is the same as multiplying by $$1$$
And we know:
$$60$$ minute = $$1$$ hour. So multiplying something by:
$$ \dfrac{1\ hr}{60\ min} $$ is the same as multiplying by $$1$$
So we can rewrite $$9,000$$ seconds as:
$$9000\ sec \times \dfrac{1\ min}{60\ sec} \times \dfrac{1\ hr}{60\ min} $$ is the same as multiplying $$9000$$ seconds by $$1$$ twice :
$$ 9000\ sec \times \dfrac{1\ min}{60\ sec} \times \dfrac{1\ hr}{60\ min} \Rightarrow $$
$$ 9000 \times \dfrac{1}{60} \times \dfrac{1\ hr}{60} \Rightarrow \dfrac{9000 \times 1 \times 1\ hr}{60 \times 60} \Rightarrow $$
$$ \dfrac{9000\ hr}{3600} \Rightarrow 2.5\ hr $$
$$9,000$$ seconds is $$2.5$$ hours.