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it will be 1,
black balls will always get finished up, because it will either come with black(then both black balls are discarded) or it will come with red(only black ball will be discarded), now red balls will be only discarded in pairs(means always even no. of red balls are discarded), now since there are 19(odd) red balls, only 18 among them will be discarded.
final content of bag at second last trial will be either (red, black) or (red, red, red) and finally in last trial bag will left with one red ball only in both the cases,
hence, probability of above experiment is 1.