Lump sum Qty errors in Variation

Dear friends

Please advise about your comments for the below mentioned secnario

Example as per Tender Drawings the Qty of foundation Concrete is 100 cum , while in the BOQ by Mistake its mentioned as 50 cum.

Now there has been changes and qty as per new drawings is  120 cum , 

Please advise what would be the qty considered for variation 

Option #1 -  Using BOQ Qty = 120-50  = 70 Cum

Option # 2 - Unsing Qty as per Tender = 120-100 = 20 cum

Please give your feedback 

Thanks