Answer:
You will have 120g Yttrium remaining
Explanation:
Since the half life is 10 hours, and you are required to find out the remaining amount after 30 hours, you can do 30 / 10 = 3.
Yttrium undergoes its half life 3 times in 30 hours.
What you can do is divide the original amount by 2 thrice.
960/2 = 480 (1st half life)
480/2 = 240 (2nd half life)
240/2 = 120 (3rd half life)
You will have 120g Yttrium remaining