The sides of the triangle are 5: 7: 11, and the sum of the largest and smallest sides is 80 cm. Count the perimeter.

1. Find the sum of the parts that correspond to the largest and smallest sides:
5 + 11 = 16 pieces.
2. Since 16 parts correspond to 80 cm, we find the length of one part:
80/16 = 5 cm.
3. Add the sum of the large and small parts to the part of the middle side:
7 + 16 = 23 pieces.
4. The sum of all parts corresponds to the perimeter, multiply the number of all parts by the length of one part, we get the perimeter of the triangle:
23 * 5 = 115 cm.
Answer: the perimeter of the triangle is 115 cm.