Answer:
5.5 min
Explanation:
At time t=0 you have 72 grams
at time t = 22 min you have 4.5 grams.
Now
4.5/72 = (1/2)^(t/half-life)
and t=22, so
4.5/72 = (1/2)^(22/half-life)
taking log of both sides:
log(4.5/72)=(22/half-life)log(1/2)
solving for half-life gives half-life = 11/2 = 5.5min