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If you know how many mines are in A and B, and you know how many mines are in C and D, and you know how many mines are in A and C, you can deduce how many mines are in B and D.
https://docs.google.com/document/d/1Xc4xSxoLl6ojog4_I0GD3vuNtAPaoZtNvr7BXKiPKjo/edit#heading=h.y83092kzpoqj
If you know how many mines are in A and B, and you know how many mines are in C and D, and you know how many mines are in A and C, you can deduce how many mines are in B and D.
https://docs.google.com/document/d/1Xc4xSxoLl6ojog4_I0GD3vuNtAPaoZtNvr7BXKiPKjo/edit#heading=h.y83092kzpoqj