The isotope radium-226 decays into radon-222, with a half-life of around 1,600 years. if a rock contained 6 grams of radium-226 when it reached its closure temperature but only 0.375 grams when it was discovered, which two statements about the rock are true?

Isotopes are atoms of the same element with the same atomic number but different mass number. unstable atoms atoms undergoes radioactive decay. Therefore radioactive decay is the process by which unstable nucleus loss energy by emitting radiations such as beta , alpha and gamma particles.
The initial mass of radium-226 is 6 grams and the new mass is 0.375 grams therefore the number of half lives undergone are 0.375 = 6× (1/2)n where n is the number of half lives. (1/2) n = 1/16
therefore n= 4. Meaning the isotope radium-226 has undergone 4 half lives
1 Half life = 1600 years
4 half lives = 1600 years × 4 = 6400 years
therefore, the rock reached its closure temperature 6400 years ago  and secondly when the rock was discovered it had 5.625 g mass of radon-222.