Which two number should be interchanged to make the given equation correct?

​$$21 \times 4 + (84 \div 7) \times 2 - 15 \times 5 + 42 = 105$$

(Note: Interchange should be done of entire number and not individual digits of a given number.)

$$21 \times 2+ (84 \div 7) \times 4 - 15 \times 5 + 42 = 105\\ \ \\42+12\times4-75+42=105\\ \ \\42 +48 -75+42= 105\\ \ \\57\not=105$$​

By option (a): Inter  change 4 and 2By option (b) Inter  change 84 and 42

​$$21 \times 4 + (42 \div 7) \times 2 - 15 \times 5 + 84= 105\\ \ \\84+6\times2- 75 +84= 105\\ \ \\84 +12-75 +84= 105\\ \ \\ 180-75= 105 \\ \ \\\bf105 = 105$$​​

Thus, correct option is (b).