approximate the age of a sample rock

Question

If the half-life of K-40 is 1.3 billion years, approximate the age of a sample rock on the island that has 90.75% of its parent K-40 remaining.

Answer 1

Age = (number of half lives)*(time of one half time)

The half-life of K-40 is 1.3 billion years.

From the graph, number of half lives = 0.15.

Age = (number of half lives)*(time of one half time)

      = (0.15)*(1.3 billion years)

      = 0.195 billion years

Age = 195 million years.

Source : http://lpc1.clpccd.cc.ca.us/lpc/hanna/HistoricalGeology/halflifexamples.pdf.