n >= 1125
First, make an equation to express the money raised after expenses.
55n - 15n + 10000 = m
The above expression can be read as "55 dollars per running is raised, but 15 dollars is spent per running to support them. Plus we get 10000 as a constant regardless of how many runner we have." Now convert that to an inequality for at least 55000 to be raised.
55n - 15n + 10000 >= 55000
Now solve for n
55n - 15n + 10000 >= 55000
40n + 10000 >= 55000
40n >= 45000
n >= 1125
So there needs to be at least 1125 runners in order to meet the goal of $55,000.