The side of an isosceles triangle is 75 mm and the perimeter is 20 cm. Find its base.

First of all, we will express the perimeter in terms of millimeters. P = 20 cm = 200 mm.
Having denoted the length of the base through x, and given that there are two equal sides of 70 mm in the triangle, we can write the equation:
200 mm = 70 mm + 70 mm + x;
200 mm = 140 mm + x;
Where is the base x = 60 mm (or 6 cm).
Answer: 60 mm (6 cm).