there·of 
/ðeɚˈʌv/ adverb
/ðeɚˈʌv/ adverb formal : of the thing that has been mentioned
▪ The professor explained the problem and solution thereof. [=the problem and the solution of the problem] ▪ A will or any part thereof [=any part of a will] may be revoked by a subsequent will. ▪ The problem is money, or (a/the) lack thereof. [=the problem is a lack of money]
▪ The professor explained the problem and solution thereof. [=the problem and the solution of the problem] ▪ A will or any part thereof [=any part of a will] may be revoked by a subsequent will. ▪ The problem is money, or (a/the) lack thereof. [=the problem is a lack of money]
