The function f(x) varies inversely with x and f(x)=250 when x=50. what is f(x) when x=5

First you need to find your k

f(x) = k / x
250 = k / 50
50 * 250 = k
12500 = k

Now that we have found k we can find f(x) when x = 5
f(x) = k / x
f(x) = 12500 / 5
f(x) = 2500