jmc

We have $20=\frac{5}{9}(F-32)$. Multiplying both sides by 9 to get rid of the fraction gives $$9\cdot 20=9\cdot \frac{5}{9}(F-32),$$ so $180=5(F-32)$. We could distribute on the right-hand side, but it's a bit faster to divide both sides by 5, giving $36=F-32$. Adding 32 to both sides gives $F=\boxed{68}$.