Let's represent Thomas's age as T and Hulian's age as H. According to the problem, Hulian is 15 years younger than Thomas, so we can write the equation:
H = T - 15
The sum of their ages is 31, so we have:
H + T = 31
Now, we can substitute the first equation into the second equation:
(T - 15) + T = 31
Combine the terms:
2T - 15 = 31
Add 15 to both sides:
2T = 46
Divide by 2:
T = 23
Thomas's age is 23 years old.