Validation on Percent Field and Text Area field #inSalesforce

Validation on Percent field:

If the percent is 25% that you want to check then use theField__c >= 0.25

Validation on Text Area field is blank:

LEN(theField__c)=0

Example:

In Opportunity,

if Probability (the standard field suppose) >= 0.25 then the sampleDecsription__c should have a value.

Use this here-

AND(Probability >= 0.25,LEN(sampleDecsription__c)=0)

Here Probability is percent field and sampleDecsription is Text Area field