1. The problem statement, all variables and given/known data The problem is stated here (from brilliant.org) http://i.imgur.com/MkWwnjT.png 2. Relevant equations momentum = mass * velocity 3. The attempt at a solution My attempt at a solution can be found here: http://i.imgur.com/Zsco86W.jpg I get the answer 0.035kg, but that is not an option. What am I doing wrong?