To calculate the perimeter of the quadrilateral and the length of the flashing light cord used to decorate the boat, we proceed as follows:

### 2. Perimeter of Quadrilaterals

**a) For the quadrilateral with sides:**
- \(3 \, \text{dm}\)
- \(4 \, \text{dm}\)
- \(5 \, \text{dm}\)
- \(6 \, \text{dm}\)

**Solution:**
\[
\text{Perimeter} = 3 \, \text{dm} + 4 \, \text{dm} + 5 \, \text{dm} + 6 \, \text{dm} = 18 \, \text{dm}
\]

**b) For the quadrilateral with sides:**
- \(10 \, \text{cm}\)
- \(15 \, \text{cm}\)
- \(10 \, \text{cm}\)
- \(15 \, \text{cm}\)

**Solution:**
\[
\text{Perimeter} = 10 \, \text{cm} + 15 \, \text{cm} + 10 \, \text{cm} + 15 \, \text{cm} = 50 \, \text{cm}
\]

### 3. Length of Flashing Light Cord

The flashing light cord decorates the boat covering the following segments:
- \(25 \, \text{cm}\)
- \(60 \, \text{cm}\)
- \(40 \, \text{cm}\)

**Solution:**
\[
\text{Total Length} = 25 \, \text{cm} + 60 \, \text{cm} + 40 \, \text{cm} = 125 \, \text{cm}
\]

So, the length of the flashing light cord is \(125 \, \text{cm}\).