After your opponents bet there is 60 in the pot.

Say opponent folds y% of the time and calls (1 - y)% of the time

y% of the time you win 60.
(1-y)% of the time you bet 80 to win 120 (60 in pot plus his 60 to call)

Assuming this is on the flop so that you meant we can hit 9 outs on turn (if we're on the turn you adjust this bit), then we're a 2-1 dog to hit our flush.
So lets say that means we hit 33% of the time (rounding down)

So EV of bet when called is 0.33*120 - 0.67*80 = 39.6 - 53.6 = -14

So EV of move is
y*60 - 14*(1-y)
which equals 0 if and only if
60*y = 14 - 14*y
74*y = 14
y = 19% (rounded up)