How much will be left after 105 days?

102 grams would be remaining after a total of 3 half lifes occur in 105 days

There will be 102 grams left.

Since the half-life is 35 days, to figure out how many half-lives to go through, you diving 105 (the number of days total), by 35 (the number of days in a half-life). This gives us 3.

Now, you divide the base number (816 grams) in half. You do this the number of times we got earlier (in this case, three times.)

And that leaves us with 102 grams.

I hope this helps!