Có hai hộp cùng chứa các viên bi. Hộp thứ nhất có 6 viên bi đỏ và 7 viên bi xanh. Hộp thứ hai có 5 viên bi đỏ và 8 viên bi xanh. Từ mỗi hộp lấy ra ngẫu nhiên 1 viên bi. Tính xác suất để 2 viên bi lấy ra cùng màu xanh.
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Số cách lấy ngẫu nhiên từ mỗi hộp lấy ra một viên bi là \(C_{13}^{1}.C_{13}^{1}=169\) (c).
Số cách lấy ra 2 viên bi cùng màu xanh là \(C_{7}^{1}.C_{8}^{1}=56\)
Vậy xác suất để lấy ra 2 viên bi cùng màu xanh là: \(\frac{56}{169}\)
Chọn A.
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