1. Install Webform Calculator (webform_calculator) module and all required modules as usual (https://www.drupal.org/documentation/install/modules-themes/modules-7).
2. Create webform node.
3. Create few webform fields of type textfield or number.
4. Create formula webform field.
5. View this webform and fill newly added fields.

Available operators: +, -, *, /, (, ), %