i) Suppose that the life of a certain type of electron tube has an exponential distribution with a mean life of 500 hours. Find the probability that it will last for another 600 hours if the tube has been in operation for 300 hours.
ii) For a random variable X , E(X)=10 , and Var(X)=25. Find the positive values of a and b such that Y=ax-b has expectation zero and variance 1.
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