"10 students are 25% of the track team" can be turned into the question "10 is 25% of what number?" or "25% of what number is 10?"
We can translate that into the equation:
25% • x = 10 (using x to represent the unknown number)
0.25x = 10 (divide both sides by 0.25)
x = 40
There are 40 people on the track team.