On facebook I found this little survey. The answers were 192, 20, 512, 20, 20, 20, 20, 20, 16x+4, 20, 20.
The majority answered 20 (8 out of 11). This is 72.7% and pretty close to the predicted 73%, so there is no doubt that 20 is correct.
Anyway, I enjoyed that rather silly kind of a test for an other reason, and as everyone may have guessed already my answer was among the wrong ones. The difference is that I interpreted “x” as a ‘variable named x’ and not as ‘times’.
Here is my explanation. The letter “x” in the question is a typical letter printed in sherif font as used for variables. Mathematical operators are without sherif. So it has to be concluded that this is not an operator—times in this case—but a variable.
This problem is better illustrated in the next picture. The first line shows the line as displayed in the question. The second line uses the times operator. For clarity, mathematicians often use an italic sherif font for variables as shown in line 3.
Moreover, in real mathematics, times operators as in this question are forbidden between natural numbers. They are reserved for matrices and vectors. If an operator is used between natural numbers it is a dot, but mostly the operator is simply omitted, among variables in particular.
Conclusively my answer was the only mathematically correct one. Regrettably, I think nobody would believe me as true is only what most people believe in.