(263.1 years) x (half-life / 87.7 years) = 3 half-lifes
In 3 half-lifes, (1/2 x 1/2 x 1/2) = 1/8 of the atoms in a sample remain unchanged.
That's 12.5% that remain unchanged.
The other 87.5% of the atoms in the sample have decayed, and changed to either other isotopes of plutonium or into atoms of other elements.
