Answer is: 1/16 of the original mass remains unchanged after 10 years
Take 100 atoms:
After first half-life (2.5 years): 50% · 100 ÷ 100% = 50 or 1/2.
After second half-life (5 years): 0.5 · 50 = 25 or 1/4.
After third half-life (7.5 years): 0.5 · 25 = 12.5 or 1/8.
After fourth half-life (10 years): 0.5 · 12.5 = 6.25 or 1/16.
6.25 ÷ 100 · 100% = 6.25% or 1/16.
