Consider the following sets.U = {all triangles}E = {x|x ∈ U and x is equilateral}I = {x|x ∈ U and x is isosceles}S = {x|x ∈ U and x is scalene}A = {x|x ∈ U and x is acute}O = {x|x ∈ U and x is obtuse}R = {x|x ∈ U and x is right}Which is a subset of I?

An equilateral triangle has three equal sides, so it also has at least two equal sides and is therefore isosceles. Thus every member of E is in I, making E a subset of I. The other choices are not subsets: scalene triangles have no equal sides, and acute or right triangles can have different side-length classifications.

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