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What will be final velocity of three charges $q$, $q$, $2q$?
How can I reopen it? All concepts come out from questions. If I know how to solve it, I would know concept automatically.
Asking how to solve problems like that is not on topic here. If you want the question reopened, you'll need to make it ask something else. Identify the specific issue that is blocking you from getting to the solution, and ask about that.