A = 1 + 2 + 3 + 4 + ...... + 15
A = 15 × ( 1 + 15 ) : 2
A = 15 × 8
A = 120
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B = 10 + 20 + 30 + ....... + 150
B = 10 × ( 1 + 2 + 3 + 4 +....... + 15 )
B = 10 × 120
B = 1 200
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C = 1 000 + 2 000 + 3 000 + ....... + 15 000
C = 1 000 × ( 1 + 2 + 3 + ....... + 15 )
C = 1 000 × 120
C = 120 000
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A + B + C = 120 + 1 200 + 120 000
A + B + C = 121 320
121 320 : 90 = 1 348
A + B + C = 1 348 × 90 → divizibil cu 90
