Find the difference between the sum of physics lectures taken by teachers B and C together and the sum of chemistry lectures taken by teachers A and D together. 

For A Total lectures taken = 16% of 250 = 40 Physics lectures taken = 20% of 40 = 8 Chemistry lectures taken = 40 – 8 = 32 For B Total lectures taken = 20% of 250 = 50 Physics lectures taken = Chemistry lectures of A = 32 Chemistry lectures taken = 50 – 32 = 18 For C Total lectures taken = 36% of 250 = 90 Let chemistry lectures taken = x
Physics lectures taken = x + 20 According to question,
x + (x + 20) = 90 2x + 20 = 90
2x = 70
x = 35 Chemistry lectures taken = 35
Physics lectures taken = 55 For D Total lectures taken = 28% of 250 = 70 Physics: Chemistry = 3:4 Total parts = 7
7 parts = 70 Physics lectures taken = 30
Chemistry lectures taken = 40     Physics lectures taken by B and C together = 32 + 55 = 87 Chemistry lectures taken by A and D together = 32 + 40 = 72 Difference = 87 – 72 = 15