From the question it is given that,
Perimeter of rectangle = 40 cm
Length of a rectangle is three time its breadth = 3b
We know that, perimeter of rectangle = 2 × (length + breadth)
40 = 2 × (3b + b)
50 = 2 × 4b
40 = 8b
40/8 = b
b = 5 cm
Therefore, width of rectangle = 5 cm
Length of rectangle = 3b
= 3 × 5
= 15 cm