The minimum amount of tooth demineralization required for radiographic detection of dental caries is: (A) 20%, (B) 40%, (C) 60%, (D) 80%
Answer is B. Lesions confined to enamel may not be evident until approximately 30% to 40% demineralization has occurred. For this reason, the actual depth of penetration of a carious lesion is often deeper than seen in the image.
 
Oral radiology by White & Pharaoh 7th ed. Page no 288