Answer
a). $0.555$
b). $11$%
Work Step by Step
a). $F=1.8C+32$
$C=\frac{F-32}{1.8}$
However, at high temperatures, $F-32=F$
So, $C=\frac{F}{1.8}=0.555F$
Hence, the exact fraction is $0.555$
b). % difference = $\frac{0.555-0.5}{0.5}\times100=11$%