Answer: Todd used 27 black tiles.
Step-by-step explanation:
Let x represent the number of black tiles that Todd used.
Todd uses 21 white tiles & some black tiles to make a mosaic. This means that the total number of white and black tiles that Todd used is 21 + x
Each tile has an area of 3 square centimeters. This means that the total area covered by the white tiles is
21 × 3 = 63 square centimeters
Also, the total area covered by the black tiles is
3 × x = 3x square centimeters
The mosaic has a total area of 144 square centimeters. This means that
3x + 63 = 144
3x = 144 - 63
3x = 81
x = 81/3 = 27
