Maximum likelihood estimator of the ratio

Please help me is this statistical problem.

An urn contains black and white balls. A sample of size n is drawn with replacement. What is the maximum-likelihood estimator of the ratio *R* of black to white balls in the urn?

Suppose that one draws balls one by one with replacement until a black appears.

Let X be the number of draws required (not counting the last draw).

This operation is repeated *n* times to obtain a sample X1, X2, ..., Xn.

What is the maximum-likelihood estimator of *R* on the basis of this sample.