The task is to generate the correct answer for a given math problem.

Q: Problem: a man purchased earphone on friday . but he forgot how much it cost . the data he have are mean expenditure of all 7 days is 500 . expenditure on mon , tue , wed , thurs , sat , sun are 450600 , 400500 , 550300 . on friday he purchased 3 things one earphone , one pen and one notebook . pen cost 30 and notebook cost 50 . then what is cost of earphone ?

A: 620
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Q: Problem: the sum of the even numbers between 1 and n is 89 * 90 , where n is an odd number , then n = ?

A: 179
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Q: Problem: if s and t are both odd prime numbers ands < t , then how many different positive integer factors does 2 sthave ?

A:
8
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