How much drug should be used in preparing 50mL of solution such that 5 mL diluted 500mL will yield 1:1000 solution?

Examples:
How much silver nitrate should be used in preparing 50 mL of a solution such that 5 mL diluted to 500 mL will yield a 1 : 1000 solution?

1 : 1000 means 1 g of silver nitrate in 1000 mL of solution
1000 mL = 1 g 500 mL X g
X = 0.5 g of silver nitrate in 500 mL of diluted solution (1 : 1000), which is also the amount in 5 mL of the stronger (stock) solution, since the 50 mL and the 5 mL are the same strength.

5mL/50ml = 0.5g/Xg

X = 5 g, answer

How much (in milliliters) of a 17% w/v concentrate of benzalkonium chloride should be used in preparing 300 mL of a stock solution such that 15 mL diluted to 1 L will yield a 1 : 5000 solution?
1 L = 1000 mL
1 : 5000 means 1 g of benzalkonium chloride in 5000 mL of solution.
X = 0.2 g of benzalkonium chloride in 1000 mL of diluted solution (1 : 5000), which is also the amount in 15 mL of the stronger (stock) solution to be prepared, because the amount of drug has not changed but simply been diluted.
X = 4 g of benzalkonium chloride.
A 17% w/v concentrate contains 17 g/100 mL. Therefore:
17 g = 100 mL 4g XmL
X = 23.5 mL, answer
5mL =0.5g 50mL Xg
X = 5 g, answer

The answer options is d