The probability that bolt is long when one select the.first box is 60/(60+40)=60/100=3/5 as the ratio of favorable events to the total number of events. For the second box this probability is 10/(10+20)=1/3. Probability to select the first box is 1/2, and probability to select the second box is 1/2 too. We are satisfied with both outcomes (the probabilities add up) if only the long bolt will be chosen.
P(long bolt)=(1/2)*(3/5)+(1/2)*(1/3)=3/10+1/6=(9+5)/30=14/30=7/15.
Answer:The probability that bolt will long is 7/15.
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