If the half-life is 17 day then in 17 days 16/2 g of Phosphorous would have decayed leaving behind 16/2 g. This means 8 grams are left.
After another 17 days (making a total of 34 days), 8/2 g would have decayed leaving behind 8/2 g. This means 4 grams are left,
After another 17 days (making a total of 51 days), 4/2 g would have decayed leaving behind 4/2 g. This means 2 grams are left.
After another 17 days (making a total of 68 days), 2/2 g would have decayed leaving behind 2/2 g. This means 1 grams are left.
Hence it would take 68 days for 16g of phosphorus to decay to 1g