Please someone help me with a radioactivity question

Dec 2016
1
0
Rio de Janeiro
A sample of uranium ore was analyzed and it was found 51.09% of 238 92 U and 2.542% of 206 82 Pb in mass. If the half-life of uranium 238 is 4.5 * 10^9 years old, what is the age of the original stone?

The answer is 3.38 * 10 ^8 years old and I really don't know why.

Thank you m8s!:(