Answer:
5/the number of socks he has
You can fill up 4 tanks because 20 divided by 5 is 4
Using a simple guess and check method, i started with the lowest common multiple of 3, and 27 (the total growth) and used 3. if you plug i the numbers, we will say a is the dwarf lemon tree, b is the semi-dwarf lemon tree, and c is the full lemon tree, you know 3times2 equals 6. if you triple 6 you get 18. so if A grew 3 inches , B had to have grown 6 inches, and C had to have grown 18 inches. add them all and you get 27. so the dwarf lemon tree grew 3 inches, semi dwarf grew 6 inches, the full size grew 18 inches.
Y=(1/5)^x would be the only equation demonstrating decay of these