I used NDsolve for a first order ODE which results in an Interpolating Function for y[x]. My question is how do I use the values of this function y[x] in some other function f(y[x])? I do not really care what the values are for f(y[x]), but rather just need to plot this function.
Solution found. Use First[Evaluate [interpolating function] when including it in another function.

ifun = NDSolve[ODE, y[0] == 0.1}, y, {x, 0, 5}, Method -> "ExplicitRungeKutta"]

Plot[Evaluate[y[x] /. ifun], {x, 0, 5}, AxesLabel -> {"X", "Y[X]"}]

Other Function with ifun = (200x)/y[x]

Plot[Other Function with ifun, {x, 0, 5}]