Two identical squares are connected side-by-side to form one large rectangle. Each individual square has a side length of 5 centimetres.
What is the total perimeter of the new rectangle?

Answer: B. 30 centimetres

When we push the two squares together, the new rectangle has 6 outer side segments that make up its perimeter:
Top & Bottom: Each is made of two square sides, so they are 10 cm long each (10 + 10 = 20 cm).
Left & Right: These are just the single outer sides of the squares, so they are 5 cm each (5 + 5 = 10 cm).
Total perimeter = 20 + 10 = 30 cm!

Answering after opening this box earns no Practice Points.