My solution:
Total of 5 tests --> 89 x 5 = 445
Eng + Chi + Art --> 92 + 95 + 86 = 273
Math + Sci --> 445 - 273 = 172
The statement "Science score was 15% higher than his Math score" means...
... If Math score is 100 units, Science score would be 115 units.
Math and Sci (in terms of units) --> 100 + 115 = 215 (units)
215 units --> 172 marks
100 units (Math score) --> 172 / 215 x 100 = 80 (marks)
