What percent of a cesium-131 sample remains after 60 days?

Question

Cesium-131 has a half-life of 9.7 days. What percent of a cesium-131 sample remains after 60 days?

I got 1.8 but the answer is 1.4.

I just want to see my error

Answer 1

There are a couple of formulas that one can use for this type of problem. I find it easiest to use

Fraction remaining = 0.5ⁿ where n = # of half lives that have elapsed

# of half lives elapsed = 60 days x 9.7 days/half life = 6.186 = n

Fraction remaining = 0.5^6.186 = 0.01373

Percent remaining = 0.01373 x 100 = 1.37% = 1.4%