From Sun Tzu: 'If you know your enemy and know yourself, you will not be defeated in a hundred battles' — meaning understanding both your opponent and yourself prevents defeat.
彼を知り己を知れば、百戦殆からず、という言葉は戦略だけでなくビジネスにも当てはまる。
The saying 'If you know your enemy and know yourself, you will not be defeated in a hundred battles' applies not only to strategy but also to business.
試験対策でも、出題傾向（彼）と自分の弱点（己）を分析すれば有利だ。彼を知り己を知れば、百戦殆からずだ。
Even for exam prep, analyzing the tendencies of the questions (the 'enemy') and your own weaknesses (the 'self') gives you an advantage — know them both and you won't be defeated.