Question 435700
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Hi
If I am understanding Your question properly:
 100*72 = 7200 Total scores for the class
total scores for the boys = 70*75 = 5250
  7200-5250 = 1950 scores remaining for the 30 girls
   1950/30 = 65 mean mark for the girls. 

CHECKING our Answer***
 .7*75 + .3*65 = 72