What if 50mg of actinium-226 disintegrates over a period of 87hours, how many mg of actinium-226 will return?

The half life of Actinium 226 is about 1.2 days (Check with the source of the question). Let the half life be T_1/2.
The equation of the radioactive decay is
N(t)=N_0*exp(-ln2*t/T_1/2)
For the mass
m(t)=m_0*exp(-(ln2)*t/T_1/2)


There remains 6.16 mg after 87 hours.