You are given a mathematical expression with basic operations. However, the symbols used are not their usual meaning. You must follow these symbol substitution rules: ‘+’ means ‘−’ ‘−’ means ‘+’ ‘×’ means ‘÷’ ‘÷’ means ‘×’ 120 ÷ 6 × 5 + 10 − 4 = ?

Correct — B.
Rule: swap the signs first: + → −, − → +, × → ÷, ÷ → ×.

120 ÷ 6 × 5 + 10 − 4 becomes
120 × 6 ÷ 5 − 10 + 4

120 × 6 = 720 (× and ÷ left to right)
720 ÷ 5 = 144
144 − 10 + 4 = 138 → option (b)

Why the others are wrong

• (a)
  85 — 85 would need 59 taken from 144. The decoded tail is − 10 + 4, which lowers 144 by 6, not 59.
• (c)
  100 — 100 is 120 ÷ 6 × 5 worked with the original signs, before any swap and without the + 10 − 4 tail. Decoded, that front part is 120 × 6 ÷ 5 = 144.
• (d)
  98 — 98 would need 46 taken from 144. After decoding, the tail − 10 + 4 takes away only 6, leaving 138.

In a symbol-substitution item, decode first, then calculate. Rewrite the whole expression with the new signs before working any part of it, so no original sign slips through.

Then apply BODMAS: × and ÷ before + and −, and within each pair left to right. Here 120 × 6 ÷ 5 gives 144 in either order, and − 10 + 4 leaves 138.

Key facts

• Decoded, the expression reads 120 × 6 ÷ 5 − 10 + 4.
• × and ÷ rank equal and are worked left to right, before + and −.
• Worked with the original signs, the expression gives 106, which is not an option.

Common traps

• Stopping at 120 ÷ 6 × 5 = 100 with the original signs and choosing 100.
• Swapping + and − but not × and ÷, which gives 100 − 10 + 4 = 94.

No directly related past PYQ was found.