Validation on Radio field contingent on entry on number field

Validation on Radio field contingent on entry on number field

Hi,
 
Can anybody help advice how I can validate that a radion field is selected if a number field is entered.
 
For example:
- The form has two fields. One number field and another radio field
- The number field is not a required field
- The radio field is required only if a value is entered in the number field
 
Please help. Thanks!
 
regards,
KM