Solve for $k$. \[ C=\frac{k+r}{2} \]

Step 1 ：Multiply both sides of the equation by 2: \(2C = k + r\)

Step 2 ：Subtract r from both sides of the equation to isolate k: \(k = 2C - r\)

Step 3 ：The solution for k in terms of C and r is \(\boxed{k = 2C - r}\)