This question is essentially a 'Weighted Average" question, so you can solve it in a few different ways. There is a built-in 'shortcut' in these types of questions that is worth noting (since it can save you some time). As long as the RATIO of the items is the same as what is described in the prompt, the amount of each item doesn't matter.
Here, we're told that we have 15kg of the first alloy and 35kg of the second. That is a ratio of 15:35, which reduces to 3:7. As long as THAT ratio is maintained, we will get the exact same answer as we would if we use 15kg and 35kg.
So, if we had 3 kg of the first alloy and 7kg of the second, the math would be a little easier. We'd have....
[3(.12) + 7(.08)]/(3+7) =
(.36 + .56)/(10) =
GMAT assassins aren't born, they're made,
Official Guide 2016 Question Breakdown:
GMAT Club Verified Reviews for EMPOWERgmat & Special Discount
EMPOWERgmat Podcast - A Wild Secret About The GMAT Algorithm