A box has 100 blue balls, 50 red ballss and 50 black balls. 25% of blue balls and 50% of red balls are taken away. Then, percentage of black balls at present is?

After removing 25% of blue balls,
Total blue balls left = 75% of 100
= (75/100) x 100 = 75
After removing 50% of red balls, total red balls left = 50% 0f 50 = (50 x 50) / 100 = 25
∴ Required percentage = [50/(75 + 25 + 50)] x 100
= (50/150) x 100
= 33¹/₃ %