What savings rate from real income (Spreadsheet 21.3) will produce the same retirement annuity as a 15%

Question:

What savings rate from real income (Spreadsheet 21.3) will produce the same retirement as a 15% savings rate from nominal income?
Annuity
An annuity is a series of equal payment made at equal intervals during a period of time. In other words annuity is a contract between insurer and insurance company in which insurer make a lump-sum payment or a series of payment and, in return,...