The answer is 60 (wow this was a fast answer wish people answered my questions this fast)
A half-life of the isotope is 15 days and at start its mass is 32 g.
A table:
t: m:
0 32 g
15 16 g
30 8 g
45 4 g
60 2 g
75 1 g
In the first column is used an arithmetic sequence ( a 1 = 0, d = 15 ).
In the second column is used a geometric sequence ( a 1 = 32, r = 1/2 )
Strength [is proportional to] d^2
strength1/(d1)^2 = strength2/(d2)^2
800kg/(2cm)^2 = strength2/(3cm)^2
strength2 = 800 kg * (3 cm)^2/(2 cm)^2
strength2 = 800 kg * 3^2/2^2
strength2 = 800 kg * 9/4
strength2 = 1800 kg
The decimal representation of any number is a linear combination of powers of 10. In other words, given a number like 123.456, we can expand it as
for any 
, so the above is the same as
Similarly, we can write
Now it's a question of reducing the fraction as much as possible. We have 
so
