Let \( M \) be the number of students taking a math course, \( L \) be the number of students taking a language course, and \( B \) be the number of students taking both courses.
From the problem, we know:
- Total seniors \( = 300 \)
- Students taking math \( M = 140 \)
- Students taking language \( L = 145 \)
- Students taking either math or language \( = 75% \) of total seniors = \( 0.75 \times 300 = 225 \)
Using the principle of inclusion-exclusion for the number of students taking either a math or language course, we have:
\[ M + L - B = \text{number of students taking either course} \]
Substituting in the known values gives us:
\[ 140 + 145 - B = 225 \]
Now, simplifying that equation:
\[ 285 - B = 225 \]
Solving for \( B \):
\[ B = 285 - 225 \] \[ B = 60 \]
Thus, the number of students taking both a math and a language course is \( \boxed{60} \).