## Thinking Mathematically (6th Edition)

Published by Pearson

# Chapter 5 - Number Theory and the Real Number System - 5.1 Number Theory: Prime and Composite Numbers - Exercise Set 5.1 - Page 256: 69

#### Answer

$d=8$

#### Work Step by Step

A number is divisible by 9 if the sum of its digits is a multiple of 9. Add the digits of the number to obtain: $=1+2+3+4+d \\=10+d$ Since $0 \le d \le 9$, then the only possible value of $10+d$ that is a multiple of 9 is 18. Thus, $d$ must be 8.

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