Answer:the number of black tiles that Todd used is 27
Explanation:
Let x represent the number of black tiles that Todd used.
Todd has 21 white tiles and some black tiles to make a mosaic. Each tile has an area of 3 square. It means that the total area of the white tiles is 21 × 3 = 63 cm^2
The total area of the white tiles is x × 3 = 3x cm^2
The mosaic has a total area of 144 Square centimeters. It means that
3x + 63 = 144
3x = 144 - 63 = 81
x = 81/3 = 27