Chemical Kinetics and Nuclear Chemistry

A sample initially contains only U−238 isotope of uranium. With time, some of the U−238 radioactively decays into Pb−206 while the rest of it remains undisintegrated.
When the age of the sample is P×108 years, the ratio of mass of Pb−206 to that of U−238 in the sample is found to be 7. The value of P is ______ .
[Given : Half-life of U−238 is 4.5×109 years; loge2=0.693 ]