What will come in place of the question mark (?) in the following equation, if ‘+’ and ‘–’ are interchanged and ‘×’ and ‘÷’ are interchanged? 6408 × 8 + 27 – 15 ÷ 10 = ?

Correct — C. Swap the symbols first, then apply BODMAS to the rewritten line.

After the interchange the equation reads 6408 ÷ 8 − 27 + 15 × 10.

6408 ÷ 8 = 801
15 × 10 = 150
801 − 27 = 774
774 + 150 = 924 → option (c)

Why the others are wrong

• (a)
  922 — 922 needs the first term to be 799. The division is exact: 8 × 800 = 6400, and the remaining 8 makes 6408 ÷ 8 = 801.
• (b)
  923 — 923 is what you get by stopping the division at 800 and letting the last 8 of 6408 go. The true quotient 801 lifts the total to 924.
• (d)
  921 — 921 comes from adding 147 at the closing step. The interchange turns 15 ÷ 10 into 15 × 10 = 150, so the last sum is 774 + 150.

This is a symbol-substitution sum, not an arithmetic sum. Two things must happen, in order.

First replace every symbol by the one it swaps with, so + becomes − and − becomes +, while × becomes ÷ and ÷ becomes ×.

Only then does BODMAS apply to the rewritten line: division and multiplication settle before addition and subtraction.

Candidates who compute strictly left to right after the swap get a different number every time, because the order of operations is what the item is really testing.

Key facts

• The swap is applied to the printed symbols, never to the numbers or their order.
• BODMAS runs on the rewritten expression, so 6408 ÷ 8 and 15 × 10 are settled before 801 − 27 + 150.
• 6408 ÷ 8 = 801 exactly, because 8 × 801 = 6408.

Common traps

• Swapping the symbols and then working left to right instead of applying BODMAS.
• Swapping just one of the two pairs the question names.
• Dividing 6408 by 8 as 800 and losing the final unit.

No directly related past PYQ was found.