If t=1600 and A=initial amount of radium, then after 1600 years, the amount will be A/2, and in 3200 years it will be A/4, in 4800 years it will be A/8 and so on. So the exponent of 2 is -t/1600. A(t)=A×2^(-t/1600) or A×2^-0.000625t. A(1)=0.999567A approx. The factor for the decay per year is 2^-0.000625=0.999567.
(And the decay after 1600 years, t=1600, is 2^-1=0.5.)
In 3200 years we have already seen this is 1/4 or 0.25.
400 years ago t=-400 so the amount is 2^0.25=1.1892 or an increase of 18.92%.