1a.)
m ( t ) = m0 * e ^{-b*t}
m ( t ) / m0 = e ^{-b*t}
m ( 1620 ) / m0 = e ^{-b*1620}
50 / 100 = e ^{-b*1620}
e ^{-b*1620} = 0.5 | ln
ln ( e ^{-b*1620} ) = ln ( 0.5 )
-b * 1620 * ln ( e ) = ln ( 0.5 )
-b * 1620 = ln ( 0.5 )
-b = -0.0004279
b = 0.0004279
m ( t ) = m0 * e^{-0.0004279*t}