Iodine-131 has a half life of about 8 days. about how much is left from a 50 gram sample after 24 days?

From the information given, we can see that the mass of Iodine-131 halves about every 8 days. There are 3 sets of 8 days in 24 days, so we will need to halve the mass of the sample 3 times:
50÷2=25
25÷2=12.5
12.5÷2=6.25
So after 24 days, the Iodine-131 sample will be 6.25g.
Here's a formula to figure this out, when dealing with bigger numbers:

Where
a = final amount
i = initial amount
1/2 is used because we are dealing with half lifes
t = time
h = half life of the isotope