CAT Exam Model Paper 6 with solutions for free online practice

Total number of tiles along any row on the floor$=\frac{39}{3}=13$.
Total number of tiles on the floor $=13\times 13=169.$
Out of these 169, number of black tiles placed along the diagonal $=2\times 13-1=25.$
Total number of tiles along any row on the floor= 39 4-3 = 13.Total number of tiles on the floor = 13 × 13 = 169.Out of these 169, number of black tiles placed alongthe diagonal $=2\times 13-1=25.$
If the white and the black tiles are needed to be placed in alternate positions then we finally require $\frac{169-1}{2}=84$ white tiles.
Total number of white tiles to be replaced by blacktiles = 144 - 84 = 60.
Alternate solution:
If we start from topmost row (1st row) a total of 5 white tiles to be replaced by black tiles. In the next row it is 4. Then again it is 5. Only exception to this alternate pattern is the row number 7 in which a total of 6 tiles should be replaced.
Hence the total tiles to be replaced$=5\times 7+4\times 6+1=60.$