Let $U$ be the set of all triangles in a plane. If $A$ is the set of all triangles with at least one angle different from $60^{circ}$, what is $A^{prime}$ ?

Question:

Let $U$ be the set of all triangles in a plane. If $A$ is the set of all triangles with at least one angle different from $60^{\circ}$, what is $A^{\prime}$ ?

Solution:

$\mathrm{A}^{\prime}$ is the set of all equilateral triangles.