Thanks for your help. But now I have weird problem. Since qx in problem is a discrete random variale 's probability distribution. Actually ,the integral expressed the meaning like following: x1 and x2 and x all follows the same distribution. I1: P(x1 > (x-10)); I2: P(x2>(x-20)); So why finialy the answer is 26500/1323? which is greater than 1? Thanks

Thanks for your help. But now I have weird problem. Since qx in problem is a discrete random variale 's probability distribution. Actually ,the integral expressed the meaning like following: x1 and x2 and x all follows the same distribution. I1: P(x1 > (x-10)); I2: P(x2>(x-20)); So why finialy the answer is 26500/1323? which is greater than 1? Thanks