Radioactivity Half Life Question

This question has been puzzling me for hours!

A sample of gas contains 8g of radon-222.
How much radon-222 will decay in 15.2 days?

You need to know the half life of radon to do this.

The half life of radon is 3.8 days. Was this given in the question?

For each half life period of 3.8 days, half of the radon will decay. (By definition)
The time interval you have is 15.2 days.
What conclusion would you draw from this?