In one of the rocks brought back from the Moon, it is found that70.7 percent of the initial potassium-40 in the rock has decayed to argon-40.
(a) If the half-life for this decay is 1.2 109 years, determine how old is the rock?
___________? y
(b) find how much longer will it take before only 8.5percent of the original potassium-40 is still present in the rock?
___________? y