The half-life of uranium-235 is 713 million years. suppose a rock originally had 26 grams of uranium-235. a geologist had the rock tested, and found that it now has only 3.25 grams of uranium-235.

approximately how old is the rock?

Explanation:

Half life of a material is the time required for the material to decay into half of initial amount.

initially there is 26 grams of uranium-235, and the final amount is 3.25 gram

                 ratio between initial and final amount =

now it is clear that the material have halved 3 times from initial condition

number of half lives passed = 3

number of half lives passed can also be found using the equation

                no of half lives =

where is the initial amount of material and R is the final amount

∵ Number of half lives =

                                      = 3

So, age of rock = half-life X number of half-lives passed

                          =713000000 X 3

                          = 2139000000

                          =2139 Million Years

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