Given Equation: 21 + 5 × 2 – 21 ÷ 3 = 12
1) + and -
After interchanging signs
LHS = 21 - 5 × 2 + 21 ÷ 3 = 21 – 10 + 7 = 18 ≠ RHS
2) – and ÷
After interchanging signs
LHS = 21 + 5 × 2 ÷ 21 – 3 = 21 + 0.476 – 3 = 18. 476 ≠ RHS
3) – and ×
After interchanging signs
LHS = 21 + 5 - 2 × 21 ÷ 3 = 21 + 5 – 14 = 12 = RHS
4) × and ÷
After interchanging signs
LHS = 21 + 5 ÷ 2 – 21 × 3 = 21 + 2.5 – 63 = - 39.5 ≠ RHS
Hence, “– and ×” is the correct answer.