If '+' means '−', '−' means '×', '×' means '÷', and '÷' means '+', then 20 + 5 × 4 − 2 = ?

Correct — D.
Rule: swap every sign first: + → −, − → ×, × → ÷, ÷ → +.

20 + 5 × 4 − 2 becomes 20 − 5 ÷ 4 × 2

5 ÷ 4 = 1.25 (÷ and × go left to right)
1.25 × 2 = 2.5
20 − 2.5 = 17.5 → option (d).

Why the others are wrong

• (a)
  12 — 12 is what 20 ÷ 5 + 4 × 2 gives, a different set of swaps. The given code turns + into −, so the expression starts 20 −.
• (b)
  5 — 5 would need the part after 20 − to equal 15. But 5 ÷ 4 × 2 = 1.25 × 2 = 2.5.
• (c)
  16.78 — 16.78 would need 5 ÷ 4 × 2 to be 3.22. But 5 ÷ 4 is exactly 1.25, so no long decimal can appear.

Symbol-substitution items give a new meaning to each operator. Replace every symbol first and write the new expression out in full. Only then calculate.

Work the new expression with BODMAS: division and multiplication before subtraction, and between ÷ and × go left to right.

Key facts

• Substitute every operator before calculating anything.
• × and ÷ rank equal: work them left to right, before + and −.
• Here 20 − 5 ÷ 4 × 2 = 20 − 2.5 = 17.5.

Common traps

• Working left to right without BODMAS: (20 − 5) ÷ 4 × 2 = 7.5
• Multiplying 4 × 2 before dividing: 20 − 5 ÷ 8 = 19.375
• Calculating with the original signs: 20 + 5 × 4 − 2 = 38

No directly related past PYQ was found.