Hey, you awesome!
It's easy, you just have to spot a reward amount with a close value to 0.420 in the decimal value of the reward. Example, 4.410, 3.419, 1.417... Make sure the decimal value of the number is closer to 0.420.
Awesome upvote and prizes await you, so go, look for 0.420 and win it!
Here is my attempt at catching 0.420. I am so close, I know, beat me to it!
UPDATE: See @easyrider's comment below. The reward has to be claimed and should be in the History. So, my screenshot is kinda too early because I was excited to catch it. LOL