Use the definition of conditional probability:
[tex]P(\text{sports}\mid\text{senior})=\dfrac{P(\text{sports AND senior})}{P(\text{senior})}[/tex]
We know that 0.10 of students belong to both categories, and that 0.25 of students are seniors, so
[tex]P(\text{sports}\mid\text{senior})=\dfrac{0.10}{0.25}=0.40[/tex]