Correct Answer: 51 and 20, – and +

Solution : Given:
20 – 10 × 342 ÷ 19 + 51 = 211

Interchange the numbers and signs in the options one by one in the given equation.
First option: 51 and 20, – and +
20 – 10 × 342 ÷ 19 + 51 = 211
⇒ 51 + 10 × 342 ÷ 19 – 20 = 211
= 51 + 10 × 18 – 20
= 51 + 180 – 20
= 211 = R.H.S.
Second option: 20 and 342, – and ÷
20 – 10 × 342 ÷ 19 + 51 = 211
⇒ 342 ÷ 10 × 20 – 19 + 51 = 211
= 34.2 × 20 –19 + 51
= 684 – 19 + 51
= 716 ≠ 211
Third option: 10 and 20, – and ×
20 – 10 × 342 ÷ 19 + 51 = 211
⇒10 × 20 – 342 ÷ 19 + 51 = 211
= 10 × 20 – 18 + 51
= 200 – 18 + 51
= 233 ≠ 211
Fourth option: 19 and 342, × and ÷
20 – 10 × 342 ÷ 19 + 51 = 211
⇒ 20 – 10 ÷ 19 × 342 + 51 = 211
= 20 – 0.53 × 342 + 51
= 20 – 181.26 + 51
= –110.26 ≠ 211

So, only the first option satisfies the R.H.S. of the given equation, when the numbers are changed. Hence, the first option is correct.