\( x \) - Plastic material worth 2.75 /kg
\( y \) - Plastic worth 3.5 /kg
\( z \) - Mixture (4 /kg)
Profit = \(\frac{\text{net worth}}{\text{Capital}} \times 100 = \frac{\text{Gross} - \text{Capital}}{\text{Capital}} \times 100 \)
\( 2.75x + 3.5y = 4z \)
\( x + y = z \)
\( 0.15 = \frac{4(x + y) - (2.75x + 3.5y)}{2.75x + 3.5y} \)
\( 0.15 = \frac{4x + 4y - 2.75x - 3.5y}{2.75x + 3.5y} \)
\( 0.4125x + 0.525y = 1.25x + 0.5y \)
\( 0.25y = 0.8375x \)
\( \frac{y}{x} = \frac{0.25}{0.8375} = \frac{2}{67} \)