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Answer: 18
Step-by-step explanation: 18=1+8=9
its complicated but when you check something like
23= 2+3 =5
10 = 1+0=1
the individual digits of 18 are 1 and 8
Sum = 1+8=9
multiply 9 by 2 we get 18
18 is also a multiple of 9.
I hope this helps