Answer:
a) $670
b) $6600
Step-by-step explanation:
a) The decrease by 9.4% means that the price of 1 kg of silver is 90.6% of the original price
0.906P = 607, P is the price of 1 kg of silver on 1st January 2015
P = 607/0.906 = 669.9779249 ≈ $670
b) The decrease of the price was $1320, which is 20%.
0.80P = P - 1320, P is the price of 1 tonne of copper on 1st January 2010
-0.20P = 1320
P = 1320/(-0.20) = $6600
