For 99% of the barium-141 to be gone, 1% (0.075mg) of the starting material remains
For 7.50mg,
after the 1st half-life 3.75mg remain
after the 2nd half-life 1.875mg remain
after the 3rd half-life 0.9375mg remain
after the 4th half-life 0.46875mg remain
after the 5th half-life 0.234375mg remain
after the 6th half-life 0.1171875mg remain
after the 7th half-life 0.05859375mg remain and so on.
So it takes 7 half-lives to reach 99% of the barium-141 gone.
7*(18.27 minutes)= 127.89 min = 2.13 hr