What is the difference between number of light red balls and number of dark yellow balls?

For Red balls: Total percentage is 21%

Total red balls = $0.21 \times 1000 = 210$ balls

Ratio of light:dark red = $3:4$

Total parts for red = $3 + 4 = 7$ parts

Light red balls = $\frac{3}{7} \times 210 = 90$ balls

For Yellow balls: Total percentage is 21%

Total yellow balls = $0.21 \times 1000 = 210$ balls

Ratio of light:dark yellow = $9:5$

Total parts for yellow = $9 + 5 = 14$ parts

Dark yellow balls = $\frac{5}{14} \times 210 = 75$ balls

Therefore, difference between light red and dark yellow balls = $90 - 75 = 15$ balls