Millie has some star-shaped tiles. Each edge of a tile is 5 centimeters long. She puts two tiles together to make a shape. We need to work out the perimeter of Millie's shape. Each star has 10 edges. When two stars are joined together, 2 edges are removed from the perimeter.

1. Problem Description

Millie has some star-shaped tiles. Each edge of a tile is 5 centimeters long. She puts two tiles together to make a shape. We need to work out the perimeter of Millie's shape. Each star has 10 edges. When two stars are joined together, 2 edges are removed from the perimeter.

2. Solution Steps

Each star has 10 edges.

Perimeter of one star = 10 * length of each edge

Length of each edge = 5 cm

Perimeter of one star = $10 \times 5 = 50$ cm.

When two stars are put together, two edges are hidden. The total perimeter of two stars is reduced by $2 \times 5 = 10$ cm, as each edge is 5 cm. Therefore, the perimeter of the combined shape is $50 + 50 - (2 \times 5) = 100 - 10 = 90$ cm.

3. Final Answer

90 cm