Chapter 2 - Set Theory - 2.2 Subsets - Exercise Set 2.2 - Page 68: 20

{F,I,N} ⊂ {F,I,N,K}

As all the elements of the first set are present in the second set but the number of elements are lesser in the first set then the second set. Therefore first set is proper subset of second set. {F,I,N} ⊂ {F,I,N,K}