Determine the set of natural numbers n if 2/5=

We'll multiply the inequality by
10:

10*2/5 =< 10*(n+1)/10 = <
10/2

We'll simplify and we'll
get:

4 =< n + 1 =<
5

We'll solve the left
inequality:

4 =< n +
1

We'll subtract 1:

n
>= 3

We'll solve the right
inequality:

 n + 1 =<
5

We'll subtract 1:

n
=< 4

The n natural  values of the set
are: {3 ; 4}.