You have to formulate equations for this problem.
 Let S = Science score
  M = Math score
  C = Chemistry score
 To illustrate the given:
 0.9S = 0.75M
 0.9S = 0.8C
 You are given that Karen’s Math score is 96 marks. You have to substitute the Math score to the first equation.
 0.9S = 0.75(96)
 0.9S = 72
 S = 80
 Therefore, Karen’s Science score is 80. Now, you have to substitute the Science score to the second equation.
 0.9(80) = 0.8C
 0.8C = 72
 C = 90
 So, Karen’s Chemistry score is 90.
 Therefore, the total score of the 3 subjects is 266 (96 + 80 + 90).