Chapter 4 - Number Representation and Calculation - 4.4 Looking Back at Early Numeration Systems - Exercise Set 4.4 - Page 241: 18

$900$

Work Step by Step

$M=1000 \\C=100$ Note that when a smaller number is written before a bigger number, the smaller number is subtracted to the bigger number. Thus, $CM = 1000-100 = 900$

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