1. Find an enumeration of the set of all odd integers

I know x/2 when x is even and -(x+1)/2 when x is odd represent natural numbers I just cannot turn this into odd numbers only.

when x is even the numbers I need are {2,6,10,14...}

when x is odd the numbers are {1,5,9,13...}

each are separated by 4 but when put together {1,2,5,6,9,10,13,14,...} I cannot find something that works like this.

2. Find an enumeration of the set of all rational numbers that have a terminating decimal representation.

(E.g. 17=10 = 1:7 and 17=8 = 2:125 are in this set, but 17=9 = 1:88888 : : : is

not.)

I'm not sure how to do this one