1 1/2 5 10 1/2 18

Need to know what is the next number please help!

Just wondering if perhaps this isn't mis-written. What you wrote was a series of 6 numbers:
1
1/2
5
10
1/2
18

If instead what you meant was a series of 4 numbers, like this:
1-1/2 (that's one and half)
5
10-1/2 (ten and a half)
18

Then it would be easy. Please verify.