3) F

Number of half-lives after 8 years = 8/4 = 2 half-lives
Number of atoms of X left after 8 years = 32×10²⁰ ÷ (2×2) = 8×10²

We know all of the X atoms decay into isotope Y.
Number of X atoms that decay = 32×10²⁰ – 8×10²⁰ = 24×10²
We know that initial number of Y atoms present were 4×10²
Number of Y atoms present after 8 years = 24+4 = 28×10²