I think there are likely two issues:
- in the input: ans section, you need to set forbid float to no;
- you need to set simp:false to preserve the form of your expression with a floating-point denominator.
Your question will look like:
N : 150.1;
ta : v/N;
Another method would be to ask for the numerator and denominator separately. I would likely do that since there are fewer pitfalls.