Need help writing an Opacity Expression that is 0 or 100 not 0 to 100

I'm using a song and converting the audio to keyframes.

I have images of a stop motion that I linked to react to the audio keyframes at certain unit levels. 

Everything works great, however my expression "linear(value,20,30,0,100)" is fading up the images via opacity from 0-100. And as you would imagine when you go frame by frame you can clearly see the fading from 0 to 100. 

Is there a way to write the same expression but instead of fading 0 to 100 the expression is either 0 or 100? Essentially the image is turned off at 0 or on at 100 no in between fades.