"what percentage of earth's total lifetime is habitable? "

The correct answer is - 82.61%

Our planet is estimated to be 4.6 billion years old, while the first life on our planet has risen around 3.8 billion years ago. So we have a situation where 3.8 billion years of Earth's geological history it has been habitable. To get to the percentage and correct result we need to multiply 3.8 (billion) by 100 (the number of total percents), and than divide the result with 4.6 (billion).

3.8 x 100 = 380

380 / 4.6 = 82.61%