At what temperature will the Celsius reading be twice as high as the Fahrenheit reading

C=(5/9)(F -32) to convert Fahrenheit to Celsius.

So when will (5/9)(F-32)= 2(F)

(5/9)F-(32*5)/9= 2F
-160/9= 2F-(5/9)F

-160/9=( 13/9)F

(-160/9)(9/13)=F

F= -160/13 or about -12.3 degrees F

Then C= (5/9)(-12.3-32)=-24.6 degrees C

BRRRRR!
Hoping this helps!