An isosceles triangle has a pair of equal sides and angles, and the third side/angle different from the other two. So, the first triangle (20-20-10) is isosceles because it has to sides with the same measure (20) and the third which is different (10)
A scalene triangle has three different angles and three different sides. So, the second triangle is scalene, because the lengths of its sides are all different.
Finally, an equilateral triangle has all three sides and angles of the same measure, so the third triangle is equilateral, because all his sides are 5 units long.
