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 $60^o$, what is A'?

$U$- Set of all triangles in a plane
$A$- Set of triangles with at least one angle different from $60^0$ i.e. the set cannot have equilateral triangles
$A^{\prime }=U-A$
So, $A^{\prime }$ is the set of all equilateral triangles.