Open the Boost Juice App and scroll down to "View Challenges”. There are a couple of challenges from $1-$2.
When someone works out the answer from the post let us know and we can update the deal here - https://www.facebook.com/162078000507169/posts/4543186572396...
Voucher expires 7 days from completion of a challenge.
Edit: Thanks to Slo20 and Bappy, deal has been updated with the correct answer which is 256.