at what temperature does the kelvin thermomrtr reads twice the fahrenheit thermometer

C = 5/9(F-32) or (F-32)/1.8

We want C = 2F or F = C/2

So C = (C/2 - 32)/1.8

1.8 C = C/2 - 32

1.3 C = -32

C = -24.6, F = -12.3

-24.6 degrees C = -12.3 degrees F. That’s rounded to the nearest tenth of a degree.