Let CP $= \text{Rs.} 100$ per $1000$ g. Then MP $= \text{Rs.} 140$ per displayed $1000$ g. SP (after discount $d$) $= 140(1-d/100)$ per displayed $1000$ g.

The trader actually delivers $800$ g per displayed $1000$ g. The CP of $800$ g $=\text{Rs.} 80$. The trader receives $140(1-d/100)$.

Given: profit $= 40$ of actual CP, i.e., profit $= 0.40\times 80 = 32$. So

$

140(1-d/100) - 80 = 32 ;\Longrightarrow; 140(1-d/100) = 112 ;\Longrightarrow; 1-d/100 = 0.8 ;\Longrightarrow; d = 20.

$

Why this is CAT-level: The trap is the ambiguity in base of profit.'' Profit measured against *displayed-weight CP* vs.\ *actual delivered CP* gives different answers. The clean reading -- profit on the goods physically delivered'' -- fixes the base. Students who default to displayed-weight CP get a different (wrong) answer.

Answer: $d = 20$.