1. One in ten say (every one in ten = plural)
2. One in ten says (each each one in ten = singular)
I don't get what you mean by 'every one in ten' and 'each one in ten'. 'Every one' and 'Each one' mean the same to me. Could you put it in other words? Also, how is #1 plural and #2 is singular? I don't see it.
Even though I don't get what you mean. Do they have the same meaning though?
Are both of these correct? Or only #3 is?
3. Two in ten say they will buy it.
4. Two in ten says they will buy it.