a. B, C, D are positive integers, and a + B = 20, a + C = 24, a + D = 22. Let the maximum value of a + B + C + d be m, and the minimum value be n, then m-n=______ ．

It is known that C = B + 4, d = B + 2, ∵ a + B + C + D = 2B + 26, and ∵ a, B are positive integers, and the minimum value of a + B = 20, ∵ B is 1, then a + B + C + D is the minimum, that is, n = 2B + 26 = 28, the maximum value of B is 19, then a + B + C + D is the maximum, that is, M = 2B + 26 = 64, ∵ M-N = 64-28 = 36

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