In one alloy there is 12% chromium while in another alloy it is 8%. 10 kg of the first alloy was melted together with 30 kg of the second one to form a third alloy. Find the percentage of chromium in the new alloy.  Choose the correct answer.
A) 9.4 % B) 9.6 % C) 9 % D) 9.8 % E) 10 %

Rationale: "the amount of chromium in the new 10 + 30 = 40 kg alloy is 0.12 * 10 + 0.08 * 30 = 3.6 kg , so the percentage is 3.6 / 40 * 100 = 9 % . answer : c"
Answer: C