Answers to activity and puzzle to illustrate how radioactive dating works:

original number of Y = 0 (because they all evaporated when the rock melted)
original number of X = 48 (current number of 6, plus the number of X's that made Y's of 42)

                                #X       #Y
after 0 years             48         0
after 1000 years       24        24  1/2 X decay
after 2000 years       12        36  1/2 X decay
after 3000 years         6        42  1/2 X decay
after 4000 years         3        45  1/2 X decay
after infinity years        0        48  infinite 1/2 lives later (long before this, the number of X would
                                                      become unmeasureable.

The rock, having 6 X and 42 Y, is 3000 years old.

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