probability - Show that if X is a continuous r.v. and it takes only positive values then $E(X) =int_{0}^{∞}Z P[X ≥ t] dt$

Show that if X is a continuous r.v. and it takes only positive values then:

$$E(X) =\int_{0}^{∞} P[X ≥ t] dt$$

I am not really sure how to begin this proof. Any help or insight would be appreciated.