Call the number 'M' (for Mystery number).
You said that two thirds of M is greater than 12 .
It starts to look like math when we write
2/3 M > 12
Multiply each side by 3 : 2 M > 36
Divide each side by 2 : M > 18 .
As long as the number is anything bigger than 18,
two-thirds of it will be bigger than 12 .