## Thinking Mathematically (6th Edition)

$d=8$
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.