$3.$ A torsion prescription balance has a sensitivity requirement of $4$ mg$.$ Explain how you would weigh $5$ mg of hydromorphone hydrochloride with an error not greater than $5\%.$ Use lactose as the diluent. $4.$ A torsion prescription balance has a sensitivity requirement of $0\mathrm{.}004$ g$.$ Explain how you would weigh $0\mathrm{.}008$ g of a substance with an error not greater than $5\%.5.$ A prescription balance has a sensitivity requirement of $6,5$ mg$.$ Explain how you would weigh $20$ mg of a substance with an error not greater than $2\%.$