Step 1: Write \(A=B+40\), \(D=0.5B\), and \(C=1.2A=1.2B+48\), all in terms of \(B\).
Step 2: Substitute into \(A+B+C+D=860\): \[ (B+40)+B+(1.2B+48)+0.5B=3.7B+88=860. \]
Step 3: Solve: \(3.7B=772 \implies B=\dfrac{7720}{37}\approx208.65\).
Step 4: Then \(A=B+40\approx248.65\), and \[ C=1.2A\approx\boxed{298.4 \text{ items}} \]
(Note: \(B\) itself is not a whole number, suggesting the given figures may not be fully consistent for whole-item production counts.)