Let x be the width.
If the length is three times its width
And a rectangle has 2 lengths and 2 widths.
2 (3x) + (1x) 2= 40
6x + 2x = 40
8x = 40
/8
x = 5
If x = 5
The length is 15
The width is 5
Formula for area = base x height
15 x 5 = 75
Answer = 75