From 1 to 20, the number 18 is the greatest number divisible by 9.To find the greatest number from 1 to 20 that is divisible by 9, we need to find the largest multiple of 9 that doesn't exceed 20.The multiples of 9 are: 9, 18, 27, 36, ...From the list, 18 is the only number that fits these three conditions:It fits between 1 and 20.It is divisible by 9.It is the highest number between 1 and 20 that is divisible by 9.