Question 5305
C=5(F-32)/9 

so 9C = 5F-160
or 9C-5F = -160     ------eq1

again C -F =0       ------eq2  as per given question

subtract (equation2 x 9)from equation 1
9C-5F =-160
-(9C-9F)= 0
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or

9C-5F =-160
-9C+9F= 0
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-5F +9F = -160
or 4F = -160
or F = (-160/4)
or F = -40

at -40F = -40C   (Ans)