FWIW, there are more than (52 * 21) = 1092 weeks in 21 years. There's more like (((365*21)+5) / 7) = 1095 weeks (actually over 1095.5 weeks; and depending on how the leap years fall there could be one more day in those 21 years). Just one of the annoying things about trying to do financial calculations on a weekly basis with rates that are yearly. (Payroll calculations for, eg, monthly allowances paid weekly are even more annoying; such things should be prohibited by the Geneva Convention.)

The formulas here (which was suggested by sherryillk) look plausible, and pretty simple to calculate (no series calculations), but I've not done enough finance to be sure of the derivation. (Hence using brute force to solve such problems.)

Ewen