Strontium90 is one of the products of the fission of uranium

Strontium-90 is one of the products of the fission of uranium-235. This strontium isotope is radioactive, with a half-life of 28.1 yr. Calculate how long it will take for 0.500 g of the isotope to be reduced to 0.150 g by decay.

1st answer = 4.43*10^17 atoms.

2 nd answer =48.823 year.