If + = −, − = +; then 20 + 8 − 5 = ?

Correct — C.
Rule: swap the signs first, then calculate left to right.

Printed: 20 + 8 − 5
After the swap (+ → −, − → +): 20 − 8 + 5

Subtract: 20 − 8 = 12
Add: 12 + 5 = 17 → option (c).

Why the others are wrong

• (a)
  23 — 23 is 20 + 8 − 5 worked with the printed signs. The stem makes + mean − and − mean +, so the swap comes first.
• (b)
  27 — 27 is not any of 20 ± 8 ± 5, which give 33, 23, 17 and 7, so no reading of the two signs produces it.
• (d)
  15 — 15 is not among 33, 23, 17 and 7, the four values of 20 ± 8 ± 5. It cannot come from keeping or swapping the signs.

Sign-interchange items give a legend, here + means − and − means +, and print the expression in code. Decode every sign in one pass, write the new expression down, then work it out.

With only + and −, the order is plain left to right. When × and ÷ join the legend, apply BODMAS after the swap.

Key facts

• Under this legend, + is read as − and − is read as +.
• 20 − 8 + 5 worked left to right is 17.
• The four sign patterns of 20 ± 8 ± 5 give 33, 23, 17 and 7.

Common traps

• Working the printed expression and picking 23
• Swapping only the first sign and getting 20 − 8 − 5 = 7

No directly related past PYQ was found.