At horse-racing competitions it is necessary to overcome four...

At horse-racing competitions it is necessary to overcome four obstacles with the probabilities equal 0,9; 0,8; 0,7; 0,6 respectively. At the first failure the sportsman in the further competitions does not participate. Compose the law of distribution of a random variable X – the number of taken obstacles. Find the mathematical expectation of the random variable X (obstacle – препятствие).