GATE 2021 EC – Question 3
The least number of squares that must be added so that the line P-Q becomes the line of symmetry is ________.
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Show answer and explanation
Correct answer: (C) 6
Explanation
Taking the grid columns on either side of P-Q, the shaded squares are: one right of the line in the top row, one two columns right in the next row, one left of the line in the third row, one two columns left and one right of the line in the fourth row, and one left of the line in the fifth row. Their mirror images are all unshaded, including the white square just left of the line in the fourth row (which is a gap in the cross shape). That gives 6 squares to add.