After 1.78 years, 1.990 g of a radioisotope remains from a sample that had an original mass of 2.000 g.

Calculate the half-life of the isotope.

ln(No/N) = kt

No = 2.000
N = 1.990
k = solve for this
t = 1.78 years.

Then substitute k into
k = 0.693/t1/2 and solve for t1/2