After one half life initial amounts gets half so :- 
- First half life :- 350÷2 = 175 grams 
- second half life :- 175 ÷2 = 87.5 grams 
- third half life :- 87.5 ÷2 = 43.75 grams 
so at last of third half life amount left = 43.75 grams 
each time amount get half 
ask ure doubt freely in comments