Question

Let EIF be the set of all natural numbers ending in 5.That is, EIF = {5, 15, 25, 35, 45, 55, 65, 75,.}.

Answer 1

N={1,2,3,4,5,6,....}
ElF={5,15,25,35,45,55,65,75....}
so let natural numbers to be n, so
a)count the distance between the numbers
b)add or subtract extra values
 10n-5 would be one to-one correspondence.