Step 1: Let the breadth of the rectangle be b cm. Since the length is double the breadth, the length is 2b cm. Step 2: The perimeter of a rectangle is 2 × (length + breadth) = 2(2b + b) = 6b. Step 3: Given perimeter = 30 cm, so 6b = 30, which gives b = 5 cm. Step 4: Length = 2b = 2 × 5 = 10 cm. So the length of the rectangle is 10 cm.