2,8:8=3,5:х

To solve the proportion \( \frac{2.8}{8} = \frac{3.5}{x} \), we can cross-multiply: \[ 2.8 \cdot x = 8 \cdot 3.5 \] Now, calculate \( 8 \cdot 3.5 \): \[ 8 \cdot 3.5 = 28 \] So, we have: \[ 2.8x = 28 \] Next, divide both sides by 2.8 to solve for \( x \): \[ x = \frac{28}{2.8} \] Calculating this gives: \[ x = 10 \] Thus, the value of \( x \) is \( 10 \).