Answer is: 0,7g.
m(Fe) = 5,60g.
t1/2 (Fe-53) = 8,51 min.
t1/2 - half-life of iron.
25.53 min/8.51 min= 3 half lives.
one half-life - mass of iron is reduced by half.
first half life - 5,60g → 2,80g.
second half life - 2,80g → 1,40g.
third half-life - 1,40g → 0,7g.