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SBI PO: 2019 Prelims examination ( Time Speed & Distance ) part-5
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Amit Kumar Gautam
Plus & Verified Educator, Experience-5years.Runs an Educational YouTube Channel--QUICK SMART EDUCATION 200k Subscribers trust Us.

Unacademy user
sir can you1 explain 2nd question once again I am not cert clear with the explanation
Ashish Bajpai
a year ago
simple logic if you want that all 3 black are taken out in rth draw , then it's means in last draw you should take out black ball , it means till r-1th draw , you should take 2 black and remaining will be white balls. Now in rth draw u will be left with 1 black and check how many are left for sample space in last draw, if you are not comfortable with r then take some specific example with r as some value then u will be able to understand concept.
Harsh Pathak
a year ago
sir its all right but what was done in last step of required probablity