How many strings of $5$ digits have the property that the sum of their digits is $7$?
Let five digit A, B, C, D, E then A + B + C + D + E = 7
given n = 7, r = 5 we know that
n + r – 1 C r - 1 = 7+5 -1 C 5-1 = 11 C 4 = 11x 10x9x8/1x2x3x4 =330
Answer B
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