Question 991505
by definition, if the operation produces even one element outside of the set, the operation is {{{not}}} closed 

set of natural numbers {{{N}}} is {{{NOT}}} closed under subtraction

example: let {{{a=3}}} and {{{b=5}}}, then {{{a-b=3-5= -2}}}  => {{{-2}}} is {{{NOT}}} element of {{{N}}}

{{{N}}} is subset of the set of real numbers {{{R}}}

note: 
Real Numbers include:
Whole Numbers (like {{{0}}}, {{{1}}}, {{{2}}}, {{{3}}}, {{{4}}}, etc)
Rational Numbers (like {{{3/4}}}, {{{0.125}}}, {{{0.333}}}..., {{{1.1}}}, etc )
Irrational Numbers (like {{{pi}}}, sqrt(3), etc )

One subset of Real Numbers is counting (or natural) numbers.  This subset includes all the numbers we count with starting with "{{{1}}}" to infinity.  The subset would look like this:

{ {{{1}}}, {{{2}}},{{{ 3}}}, {{{4}}}, {{{5}}}...}

Another subset is whole numbers.  This subset is exactly like the subset of counting numbers, with the addition of one extra number.  This extra number is "{{{0}}}".  The subset would look like this:

{ {{{0}}}, {{{1}}}, {{{2}}},{{{ 3}}}, {{{4}}}, {{{5}}}...}

so, your answer is set {{{N}}} or you can choose set of whole numbers