If you want to apply conditional validation on 'Second Number' or 'Third Number', then you need to add 'First Number' input as '25'
{{userForm.controls?.firstNumber?.errors?.factor.message}}
To apply the validation of factor on 'Fourth Number' field, then enter the value '25' in 'First Number' textbox
Conditional Expression with type 'string'
{{userForm.controls?.thirdNumber?.errors?.factor.message}}