21) The half - life of a radioactive element is 130 days, but your sample will not be useful to you after 80% of the radioactive nuclei originally present have disintegrated. About how many days can you use the sample? 22) Sandy manages a ceramics shop and uses at 650F kiln to re ceramic greenware. After turning off her kiln. she must wait until its temperature gauge reaches 160F before opening it and removing the ceramic pieces. If room temperature is 80F and the gauge reads 500F in 11 minutes, how long must she wait before opening the kiln? Assume the kiln cools according to Newton's Law of Cooling: u = T+ (uo - neki. (Round your answer to the nearest whole minute.)