Hi

there are 90 students , 60 play clarinet and 40 play the trombone, 

10 only play the trombone: that is 30 play both.

P(trombone or clarinet) = 40/90 + 60/90 - 30/90 = 70/90 = 7/9



P(A or B) = P(A) + P(B) - P(A and B) 

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