[rkowalick]

The correct calculation isn’t too hard either.

If currency halves in purchasing power in 25 yrs, that means inflation is 100% in 25 years, so

    (1 + r)^25 = 2
    r = 2^(1/25) - 1 ~ 2.8%

[nonethewiser]

In terms of being able to do it in your head, its a lot harder.