Hello,
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mistakes are common that every one does while preparing for examinations. Here are few things that needs to be considered while preparation,
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• Buttifying the topics which aren't interesting or tuff for preparation.
• Skipping the topics
• Absence of interest while preparation
• Not following a daily routine
• changes in the time table
• Most important is Only concentrating on each subject

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Now, these are the mistakes that every one does without knowing that it may effect there preparation. So, do follow a particular Time table and concentrate on every Subject.