Answer:
For temperatures between the freezing and boiling point of water, the heating curve is linear. Thus, we can use:
((200 * 90) + (100* 30))/300
Or: ((200 * 90)(temperature of first sample weighted by mass) + (100 * 30)(temperature of second sample weighted by mass) / (300)(total mass)
to find the final temperature.
100 can be factored out to get (2 * 90) + (1 * 30)) / 3
3 can be factored out every term: (2 * 30) + (1 * 10)
(The factoring out just makes mental math easier; if a calculator is available doing so is unnecessary.)
And then just work through the order of operations.
60 + 10
=70.
So in mathematics, which assumes a perfect world, you get 70.