Karen's students are about to take a standardized test. Karen gets a $500 bonus if their average score is above 75, plus an extra $10 bonus for every additional point the average score increases above 75. So far, Karen has graded 8 tests, and the average is 70. Given that each student can have a maximum score of 150, what combined score do the last two tests need to have for Karen to earn a $600 bonus?
Please reason step by step, and put your final answer within \boxed{}.
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