Note: 20 weeks = 20/52 or 5/13 of a year.
Let E = effective rate of interest
E = [1 + (r/m)]^m - 1 where r is the nominal (annual ) advertised rate; m is the number of compounding periods/year.
Let r = 0.0275
Let m = 5/13
Plug into formula and do the math.
