By Interchanging the given two numbers which of the following equation will be not correct?
1 and 9

Correct option is C

From the given options –
(a) 9 × 2 – 1 + 8 = 1 After interchanging the digit 1 and 9 we get following equation -
1 × 2 – 9 + 8 = 9
2 – 9 + 8 = 1
10 – 9 = 1
10 – 9 = 1
1 = 1
(b) 9 × 2 + 1 – 4 = 7 After interchanging the digit 1 and 9 we get following equation -
1 × 2 + 9 – 4 = 7
2 + 9 – 4 = 7
11 – 4 = 7
7 = 7
(c) 9 – 1 + 7 = -3 After interchanging the digit 1 and 9 we get following equation -
1 – 9 + 7 = -3
8 – 9 = -3
-1 ≠ -3
Correct answer is (c).