The perimeter of the quadrilateral is 60 cm, the smallest side is 15 cm shorter than the largest

The perimeter of the quadrilateral is 60 cm, the smallest side is 15 cm shorter than the largest, and the other two are equal, and their sum is equal to the largest side. What is the largest side of the quadrilateral?

Given the condition:
AB + BC + CD + AD = 60 cm.
CD – the smallest side, AD – the largest side, AB = BC.
AB + BC = AD.
Let’s designate AB and BC as x, and AD as y, then:
x + x = y;
2x = y.
By condition it is also given:
CD = AD – 15.
Substitute this expression into the perimeter formula:
AB + BC + AD – 15 + AD = 60.
x + x + y – 15 + y = 60;
2x + 2y = 75.
Since 2x = y, then:
y + 2y = 75;
3y = 75;
y = 75/3;
y = 25 cm.
AD = y = 25 cm.
Answer: AD = 25 cm.