If Mateo's number multiplied by 3 is less than 30, then the largest number that he could be thinking of is 9 (since 9 x 3 = 27). If Mateo's number multiplied by 6 is more than 40, then the smallest number he could be thinking of is 7 (since 7 x 6 = 42).
Therefore, the possible numbers that Mateo could be thinking of are 7, 8, and 9. The sum of these numbers is:
7 + 8 + 9 = 24
So, the sum of all the numbers that Mateo could be thinking of is 24.
I hope this helped!