While Alex is attending her first Spring Festival at Haven Springs in the fourth chapter of Life is Strange: True Colors, you can interact with almost all of the townsfolk and see what is going on for them. Among them is a man counting jellybeans in a jar for a competition. If anyone gets the right amount, they win a $100 gift card. Help the man get the prize, and you will earn yourself the Exactly 731 achievement/trophy. Here is what you need to do.
This achievement is really simple to get but may potentially only be obtainable if you got the Found Dog! achievement in chapter two and helped the man get his dog. If he is not at the table at the festival, go back and do that. He may be there anyway, but he mentioned trying for the card in our playthrough only because he wanted to get his dog some treats after getting her back.
After you have left Alex’s apartment and talked to Steph and Ryan the first time at the festival, you can find the jellybean counting competition to the right of the stage. First, talk to the man who is guessing in the 200s. When you get the prompt, guess 800.
After hearing you get close, the woman on the other side of the table will show some fear. Use your powers on her to find out it is closer to 700. Talk to the man again and tell him 700.
Now the woman will react again and inadvertently tell you that the exact number is 731. Talk to the man one more time to give him the winning number and win your achievement/trophy.