We use this formula to find how much of the substance is left after a certain time: N=N0(‌
1
2
)t∕T1∕2 It is said that 96.875% of the substance has decayed after 10 days. This means that only 3.125% of the substance is left, because 100%−96.875%=3.125%. We can also write 3.125% as a fraction: 3.125%=‌
1
32
. So, ‌
N
N0
=‌
1
32
. We set up the formula: (‌
1
2
)t∕T1∕2=‌
1
32
Now, ‌
1
32
=(‌
1
2
)5, so t∕T1∕2=5. So, t=5×T1∕2. We know t=10 days. So, T1∕2=‌
10
5
=2‌ days ‌ The half-life of the substance is 2 days.