Here is a simple example of how you can perform subtraction in Ansible.

---
- hosts: localhost
  tasks:
  - name: subtraction example
    set_fact:
      result: "{{ 5 - 1 }}"

  - debug:
      var: result
...

Which should return the following.

ok: [localhost] => {
    "result": "4"
}

Likewise, you should be able to work with decimal point numbers and variables, perhaps something like this.

---
- hosts: localhost
  vars:
    mynumber: 5.75
  tasks:
  - name: subtraction example
    set_fact:
      result: "{{ mynumber - 1.25 }}"

  - debug:
      var: result
...