You are given the formula, and the temp in °C .
Just write that temp in place of " °C " in the formula.
5/9 (F - 32) = 20
Multiply each side by 9 : 5 (F - 32) = 180
Divide each side by 5 : F - 32 = 36
Add 32 to each side: F = 68
If you travel a lot, or work any place where you go back and forth
between °F and °C very often, you quickly develop a feel for how
they're related.
Along the way, you learn how they line up at a few points, and
you judge everything else from these few points.
This question is probably the first point that most people learn, because
it's right smack in the middle of our daily experience: 68° F = 20° C.