Cs -137 has a half life of about 30 years. If 60 years pass, there is two half lives passed so 1/2 * 1/2= 1/4. Take 1/4 and multiply it with the mass given:
(1/4)*20mg=5mg left
The metalloid that has three valence electrons is Boron~
Answer:
2016.417222 hours
Explanation: you def made your goal
28800 seconds