Half Life for Radioactive Substance

Question

The Half-Life of a radioactive strontium-90 is approximately 31 years. In 1960, radioactive strontium-90 was released into the atmosphere during testing of nuclear weapons, and was absorbed into people's bones. How many years does it take until only 14 percent of the original amount absorbed remains?

Answer 1

0.14 = (0.5)^t/31

ln 0.14 = (t / 31)(ln 0.5)

Can you sovle for t and answer?