1. The problem statement, all variables and given/known data Three particles, A, B, and C, with masses M, 2M, and 3M respectively, lie at rest in that order in a straight line on a smooth horizontal table. The particle A is then projected directly towards B with velocity U. Assuming the collisions are perfectly elastic, I need to find the fraction of U that C moves with, immediately after impact. 2. Relevant equations 3. The attempt at a solution I used the idea that kinetic energy is conserved in a perfectly elastic collision: 0.5 * M * U^2 = 0.5 * 2M * kU^2 1 = 2k k = 1/2 Therefore the speed of B after impact is U/2 0.5 * 2M * (U/2)^2 = 0.5 * 3M *kU^2 2(U^2/4) = 3kU^2 U^2/2 = 3kU^2 1 = 6k k = 1/6 It asks for the answer as a decimal to three significant figures, so I've typed in: 1.67 x 10^-1 but it's not having it. Where have I gone wrong? Thanks for your help in advance.  I wrote it down wrong.