Answer: each piece -
 necklace = $21 
 bracelet=$10
Explanation:
each bracelet is for $21
as the price increased when 2 necklaces were sold it became 310 from 268.
so 
 price for those two necklaces would be:
310-268= $42
the price for one necklace would be 42/2 =$ 21
price for 10 necklaces = 21 X 10
 = $210
price for 10 bracelets= 310 - 210 = 100
 price for 1 bracelet = 100/10 = $10