The correct way to express the statement "1 is to 2 as 5 is to x" in fractional notation is:
Start Fraction 1 over 2 End Fraction equals Start Fraction 5 over x End Fraction
This can be written as:
\[\frac{1}{2} = \frac{5}{x}\]
So, the response would be: 12=5x (representing \(\frac{1}{2} = \frac{5}{x}\)).