The half life of an isotope means how long it will take for the amount of given isotope to decay to only 1/2 of the original amount.
If you start with 10.0g of polonium-218 and the half life is 3.0 minutes, then in 3.0 minutes you will have half of the original amount
Half of the original amount is equal to 5.0g
The answer is 5.0g