The lengths of the sides of a triangle are referred to as 3 4 5. Find the perimeter of this triangle if the longest side is 30 cm.

The sides of this triangle will be denoted by a, b and c, and also assume that the largest side is c. Then, c = 30 cm.
Let’s use the property of directly proportional quantities, which consists in the fact that the ratio of directly proportional quantities is always constant. This constant is called the coefficient of proportionality or, simply, the constant of proportionality.
According to the condition of the assignment: a / 3 = b / 4 = c / 5 = k, where k is the constant (or coefficient) of proportionality, which was discussed in paragraph 2.
We have: a = 3 * k, b = 4 * k, c = 5 * k. Since, c = 30 cm, the equality c = 5 * k allows you to determine the proportionality coefficient k = (30 cm): 5 = 6 cm.
Using this coefficient, we find the remaining sides and, of course, the perimeter of the triangle.
We have: a = 3 * k = 3 * 6 cm = 18 cm; b = 4 * k = 4 * 6 cm = 24 cm.Therefore, the perimeter of the triangle is 18 cm + 24 cm + 30 cm = 72 cm.
Answer: 72 cm.