The first thing you should know to solve this problem is the following conversion:
1 lb = 453.592 grams
Therefore, we can make the following rule of three:
453.592 grams ------> 171 $
15 grams --------------> x
Clearing x we have:
x = (15 / 453.592) * (171)
x = $ 5,655
Answer:
the cost of 15 grams is:
x = $ 5,655